<?xml version="1.0" encoding="us-ascii"?>
<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2022-03-01T17:00:13.0295199+00:00 -->
<!-- Version            : 5.0.1.728 -->
<!-- Package ID         : 0b2d8b8ae9ff48ef9fa3da71180539ca -->
<!-- Copyright (c) 2022 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="sndx_SummaryOfSignificantAccountingPoliciesTable" name="SummaryOfSignificantAccountingPoliciesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_CostsRelatedToCostSharingCollaborations" name="CostsRelatedToCostSharingCollaborations" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="sndx_IncreaseDecreaseInResearchAndDevelopmentExpense" name="IncreaseDecreaseInResearchAndDevelopmentExpense" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="sndx_PercentageOfIncomeTaxContingencyRealized" name="PercentageOfIncomeTaxContingencyRealized" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_IndemnificationObligationsClaimsReserves" name="IndemnificationObligationsClaimsReserves" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_DevelopmentCostsAssociatedWithClinicalTrials" name="DevelopmentCostsAssociatedWithClinicalTrials" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_GlobalAndUnitedStatesMember" name="GlobalAndUnitedStatesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_IncyteCorporationMember" name="IncyteCorporationMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_NonRefundableCashPaymentsMember" name="NonRefundableCashPaymentsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_LicenseAgreementMember" name="LicenseAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_KyowaKirinCoLtdMember" name="KyowaKirinCoLtdMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_UpfrontPayments" name="UpfrontPayments" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_PotentialMilestonePaymentsUponAchievementOfMilestone" name="PotentialMilestonePaymentsUponAchievementOfMilestone" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_CommercializationMilestonesAndTieredRoyalties" name="CommercializationMilestonesAndTieredRoyalties" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_UpfrontPaymentsReceived" name="UpfrontPaymentsReceived" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_CollaborationAndLicenseAgreementCashConsideration" name="CollaborationAndLicenseAgreementCashConsideration" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_CollaborationAndLicenseAgreementAllocatedConsiderationReceivedToDerivativeLiability" name="CollaborationAndLicenseAgreementAllocatedConsiderationReceivedToDerivativeLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_UpfrontPaymentsToBeReturnedUponTerminationOfAgreement" name="UpfrontPaymentsToBeReturnedUponTerminationOfAgreement" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="sndx_CollaborationAndLicenseAgreementConsiderationTransactionPrice" name="CollaborationAndLicenseAgreementConsiderationTransactionPrice" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_EstimatedStandaloneSellingPriceOfLicenseAgreement" name="EstimatedStandaloneSellingPriceOfLicenseAgreement" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="sndx_RevenueRecognizedUponTransferOfLicenseAgreement" name="RevenueRecognizedUponTransferOfLicenseAgreement" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_UpfrontPaymentAllocatedToEquityPurchase" name="UpfrontPaymentAllocatedToEquityPurchase" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_NonRefundableUpFrontPaymentReceived" name="NonRefundableUpFrontPaymentReceived" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_PotentialMilestonePaymentsReceivable" name="PotentialMilestonePaymentsReceivable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="sndx_PeriodOfTerminationAfterFirstCommercialSaleOfFirstLicensedProduct" name="PeriodOfTerminationAfterFirstCommercialSaleOfFirstLicensedProduct" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_UpfrontPaymentsAllocatedToEquityPurchaseFairValue" name="UpfrontPaymentsAllocatedToEquityPurchaseFairValue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_UpfrontPaymentAllocatedToLicenseFee" name="UpfrontPaymentAllocatedToLicenseFee" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_RevenueFromContractWithCustomerMilestoneMethodFactors" name="RevenueFromContractWithCustomerMilestoneMethodFactors" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_LeaseCommencementDate" name="LeaseCommencementDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_WarrantsToPurchaseCommonStockMember" name="WarrantsToPurchaseCommonStockMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_EmployeeStockPurchasePlanMember" name="EmployeeStockPurchasePlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_NumberOfCommonStockConvertedToWarrants" name="NumberOfCommonStockConvertedToWarrants" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_NumberOfCommonStockWarrantsIssued" name="NumberOfCommonStockWarrantsIssued" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="sndx_ClassOfWarrantOrRightIssued" name="ClassOfWarrantOrRightIssued" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_NumberOfPreFundedWarrantsExchangedForCommonStock" name="NumberOfPreFundedWarrantsExchangedForCommonStock" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_NumberOfPreFundedWarrantsCashlessExchangedForCommonStock" name="NumberOfPreFundedWarrantsCashlessExchangedForCommonStock" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_PreFundedWarrantsMember" name="PreFundedWarrantsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_BiotechnologyValueFundLPMember" name="BiotechnologyValueFundLPMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_LicenseAgreementsExpirationPeriod" name="LicenseAgreementsExpirationPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_AggregatePotentialMilestonePayable" name="AggregatePotentialMilestonePayable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_AllerganLicenseAgreementMember" name="AllerganLicenseAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_VitaePharmaceuticalsIncMember" name="VitaePharmaceuticalsIncMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_UcbLicenseAgreementMember" name="UcbLicenseAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_UCBBiopharmaMember" name="UCBBiopharmaMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_ClinicalTrialMember" name="ClinicalTrialMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_EasternCooperativeOncologyGroupMember" name="EasternCooperativeOncologyGroupMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_BayerPharmaAgMember" name="BayerPharmaAgMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_AccruedMilestoneExpenses" name="AccruedMilestoneExpenses" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_MilestonePaymentDue" name="MilestonePaymentDue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_IncreaseDecreaseInContractualObligation" name="IncreaseDecreaseInContractualObligation" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="sndx_RemainingContractualObligation" name="RemainingContractualObligation" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_ContractualObligationsTerm" name="ContractualObligationsTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_AccruedContractualObligation" name="AccruedContractualObligation" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_UpfrontLicenseFeesPaid" name="UpfrontLicenseFeesPaid" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_ComputerAndOfficeEquipmentMember" name="ComputerAndOfficeEquipmentMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_DerivativeLiabilityExpectedTimingOfTerminationRight" name="DerivativeLiabilityExpectedTimingOfTerminationRight" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_FairValueMeasurementsLineItems" name="FairValueMeasurementsLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_FairValueMeasurementsTable" name="FairValueMeasurementsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_ValuationTechniqueBlackScholesModelMember" name="ValuationTechniqueBlackScholesModelMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValueAdditions" name="FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValueAdditions" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityChangeInFairValue" name="FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityChangeInFairValue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_AvailableForSaleSecuritiesFairValueDisclosureAbstract" name="AvailableForSaleSecuritiesFairValueDisclosureAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_ReimbursableCosts" name="ReimbursableCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="sndx_AccruedClinicalCostsCurrent" name="AccruedClinicalCostsCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_CowenAndCompanyLLCMember" name="CowenAndCompanyLLCMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_AtTheMarketEquityOfferingSalesAgreementMember" name="AtTheMarketEquityOfferingSalesAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_SeriesWarrantsMember" name="SeriesWarrantsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_SeriesOneWarrantsMember" name="SeriesOneWarrantsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_SeriesTwoWarrantsMember" name="SeriesTwoWarrantsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_SeriesOneAndSeriesTwoWarrantsMember" name="SeriesOneAndSeriesTwoWarrantsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_IncyteCollaborationAgreementAndSharePurchaseAgreementMember" name="IncyteCollaborationAgreementAndSharePurchaseAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_WarrantExpirationTerm" name="WarrantExpirationTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_ExerciseOfWarrantsUpperLimitAfterExerciseAsPercentageOfCommonStock" name="ExerciseOfWarrantsUpperLimitAfterExerciseAsPercentageOfCommonStock" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_NumberOfSeriesOfWarrants" name="NumberOfSeriesOfWarrants" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_ClassOfWarrantOrRightTerm" name="ClassOfWarrantOrRightTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_OfferingPricePerShare" name="OfferingPricePerShare" type="dtr-types:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_GrossProceedsFromOffering" name="GrossProceedsFromOffering" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="sndx_OfferingCost" name="OfferingCost" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="sndx_WarrantsExercisedPeriod" name="WarrantsExercisedPeriod" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_MaximumPeriodAllowedToIncreaseBeneficialOwnershipLimitationAfterNotice" name="MaximumPeriodAllowedToIncreaseBeneficialOwnershipLimitationAfterNotice" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_ExerciseOfWarrantsLowerLimitAfterExerciseAsPercentageOfCommonStock" name="ExerciseOfWarrantsLowerLimitAfterExerciseAsPercentageOfCommonStock" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_MaximumBeneficialOwnershipLimitationPercentage" name="MaximumBeneficialOwnershipLimitationPercentage" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_FairValueOfWarrants" name="FairValueOfWarrants" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_NumberOfWarrantsExercisedInCashlessExercise" name="NumberOfWarrantsExercisedInCashlessExercise" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_TwoThousandFifteenOmnibusIncentivePlanMember" name="TwoThousandFifteenOmnibusIncentivePlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_TwoThousandSevenAndTwoThousandFifteenStockIncentivePlanMember" name="TwoThousandSevenAndTwoThousandFifteenStockIncentivePlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_ServiceBasedOptionsMember" name="ServiceBasedOptionsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_TwoThousandAndSeventeenPerformanceAwardsMember" name="TwoThousandAndSeventeenPerformanceAwardsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_EmployeesMember" name="EmployeesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_TwoThousandAndNineteenPerformanceAwardsMember" name="TwoThousandAndNineteenPerformanceAwardsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_TwoThousandAndSevenStockIncentivePlanMember" name="TwoThousandAndSevenStockIncentivePlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_ShareBasedCompensationArrangementByShareBasedPaymentAwardAnnualIncreasePercentageInNumberOfSharesAvailableForGrant" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardAnnualIncreasePercentageInNumberOfSharesAvailableForGrant" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_EmployeeServiceShareBasedCompensationRecognizedStockOptions" name="EmployeeServiceShareBasedCompensationRecognizedStockOptions" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="sndx_EmployeeServiceShareBasedCompensationRecognizedStockOptionsNotMet" name="EmployeeServiceShareBasedCompensationRecognizedStockOptionsNotMet" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="sndx_NumberOfEmployeesRetirement" name="NumberOfEmployeesRetirement" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsCanceledForfeitedOrExpiredInPeriodTotalIntrinsicValue" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsCanceledForfeitedOrExpiredInPeriodTotalIntrinsicValue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="sndx_ShareBasedCompensationArrangementByShareBasedPaymentAwardPayrollDeductionsOfferingPeriod" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardPayrollDeductionsOfferingPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOfferingPeriodDiscountRate" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOfferingPeriodDiscountRate" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="sndx_LineOfCreditFacilityElectedNotToDraw" name="LineOfCreditFacilityElectedNotToDraw" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_PaymentOfFacilityCharges" name="PaymentOfFacilityCharges" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="sndx_HerculesCapitalIncMember" name="HerculesCapitalIncMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_LoanAndSecurityAgreementMember" name="LoanAndSecurityAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_TermLoanMember" name="TermLoanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_AdditionalTermLoanMember" name="AdditionalTermLoanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_FirstAmendmentToLoanAndSecurityAgreementMember" name="FirstAmendmentToLoanAndSecurityAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_DebtInstrumentTrancheTwoMember" name="DebtInstrumentTrancheTwoMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_DebtInstrumentTrancheThreeMember" name="DebtInstrumentTrancheThreeMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_ScenarioOneMember" name="ScenarioOneMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_ScenarioTwoMember" name="ScenarioTwoMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingInFirstYear" name="PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingInFirstYear" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingInSecondYear" name="PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingInSecondYear" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="sndx_PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingThereafter" name="PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingThereafter" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_PercentageOfEndOfTermChargeToFutureDraws" name="PercentageOfEndOfTermChargeToFutureDraws" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_PercentageOfFacilityChargeOfFutureDraws" name="PercentageOfFacilityChargeOfFutureDraws" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_DebtInstrumentInterestOnlyPeriodExpirationDate" name="DebtInstrumentInterestOnlyPeriodExpirationDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_DebtInstrumentInterestOnlyPeriodExtendableUponPartialOrFullDrawOfTrancheTwo" name="DebtInstrumentInterestOnlyPeriodExtendableUponPartialOrFullDrawOfTrancheTwo" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_LineOfCreditFacilityMinimumCashCovenant" name="LineOfCreditFacilityMinimumCashCovenant" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="sndx_AdditionalDefaultInterestRate" name="AdditionalDefaultInterestRate" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_LineOfCreditFacilityMinimumCashCovenantWaived" name="LineOfCreditFacilityMinimumCashCovenantWaived" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_AmortizedLossesReacquiredDebtNoncurrent" name="AmortizedLossesReacquiredDebtNoncurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="sndx_IncomeTaxesLineItems" name="IncomeTaxesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_IncomeTaxesTable" name="IncomeTaxesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_IndefiniteCarryforwardPeriodMember" name="IndefiniteCarryforwardPeriodMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_BeginToExpireInTwoThousandAndTwentySixMember" name="BeginToExpireInTwoThousandAndTwentySixMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_DeferredTaxAssetsCapitalizedStartUpAndOtherCosts" name="DeferredTaxAssetsCapitalizedStartUpAndOtherCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="sndx_DeferredTaxAssetsCapitalizedResearchAndDevelopmentCosts" name="DeferredTaxAssetsCapitalizedResearchAndDevelopmentCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="sndx_DeferredTaxAssetsOperatingLossCarryforwardsExpirationPeriod" name="DeferredTaxAssetsOperatingLossCarryforwardsExpirationPeriod" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_DeferredTaxAssetsResearchCreditsExpirationPeriod" name="DeferredTaxAssetsResearchCreditsExpirationPeriod" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_CommitmentAndContingenciesLineItems" name="CommitmentAndContingenciesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_CommitmentAndContingenciesTable" name="CommitmentAndContingenciesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="sndx_SeriesB1PurchaseAgreementMember" name="SeriesB1PurchaseAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
</xsd:schema>
