Exhibit 5.1
February 27, 2024
Syndax Pharmaceuticals, Inc.
35 Gatehouse Drive
Building D, Floor 3
Waltham, Massachusetts 02451
Ladies and Gentlemen:
We have acted as counsel to Syndax
Pharmaceuticals, Inc., a Delaware corporation (the Company), in connection with the filing by the Company of a registration statement on Form S-8 (the Registration
Statement) with the Securities and Exchange Commission (the Commission) covering the offering of up to 4,743,065 shares (the Shares) of the Companys common stock, par value
$0.0001 per share (Common Stock), consisting of (i) 3,393,065 shares of Common Stock issuable pursuant to the Companys 2015 Omnibus Incentive Plan (the 2015 Plan), (ii) 250,000 shares of
Common Stock issuable pursuant to the Companys 2015 Employee Stock Purchase Plan (the ESPP) and (iii) 1,100,000 shares of Common Stock issuable pursuant to the Companys 2023 Inducement Plan (together with the
2015 Plan, the ESPP and the Inducement Plan, the Plans).
In connection with this opinion, we have examined and relied upon
(a) the Registration Statement and the related prospectuses, (b) the Companys certificate of incorporation and bylaws, each as currently in effect, (c) the Plans, and (d) such other records, documents, opinions,
certificates, memoranda and instruments as in our judgment are necessary or appropriate to enable us to render the opinion expressed below. We have assumed the genuineness of all signatures; the authenticity of all documents submitted to us as
originals; the conformity to originals of all documents submitted to us as copies; the accuracy, completeness and authenticity of certificates of public officials; and the due authorization, execution and delivery of all documents by all persons
other than the Company where authorization, execution and delivery are prerequisites to the effectiveness thereof. As to certain factual matters, we have relied upon a certificate of an officer of the Company and have not independently verified such
matters.
Our opinion is expressed only with respect to the General Corporation Law of the State of Delaware. We express no opinion to the extent that any
other laws are applicable to the subject matter hereof and express no opinion and provide no assurance as to compliance with any federal or state securities law, rule or regulation.
On the basis of the foregoing, and in reliance thereon, we are of the opinion that the Shares, when sold and issued in accordance with the Plans, the
Registration Statement and the related prospectuses, will be validly issued, fully paid, and nonassessable (except as to shares issued pursuant to deferred payment arrangements, which will be fully paid and nonassessable when such deferred payments
are made in full).
This opinion is limited to the matters expressly set forth in this letter, and no opinion should be implied, or may be inferred,
beyond the matters expressly stated. This opinion speaks only as to law and facts in effect or existing as of the date hereof and we have no obligation or responsibility to update or supplement this opinion to reflect any facts or circumstances that
may hereafter come to our attention or any changes in law that may hereafter occur.