<?xml version="1.0" encoding="us-ascii"?>
<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2019-08-06T16:55:42.1755164+00:00 -->
<!-- Version            : 3.6.7150.26138 -->
<!-- Package ID         : c299fe9c84d34554893c039f6811f239 -->
<!-- Copyright (c) 2019 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <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_WeightedAverageExercisePriceOfWarrants" name="WeightedAverageExercisePriceOfWarrants" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="sndx_AdjustedWeightedAverageExercisePriceOfWarrants" name="AdjustedWeightedAverageExercisePriceOfWarrants" type="num:perShareItemType" 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:schema>
