Studies in heterocycles

Abstract

The thesis entitled “STUDIES IN HETEROCYCLES: (a) SYNTHESIS OF OXYGEN AND NITROGEN POLYCYCLIC AROMATIC COMPOUNDS via o?QUINONE METHIDES AND (b) PHOTOISOMERIZATION OF SPIRONAPHTHALENONES” is presented in two chapters.

Chapter I – In Situ Generation of o?Quinone Methides and Their Reactions Chapter I, which deals with the in situ generation of o?quinone methides and their reactions, is presented in five sections. Section I contains a brief review of the chemistry of o?quinone methides and their applications in the synthesis of natural products, polycyclic aromatic compounds, etc. Reaction of 1?bromomethyl?2?(2??tetrahydropyranyloxy)naphthalene (1a) with tetrachlorocatechol in acetone in the presence of potassium carbonate resulted in the formation of novel diastereomeric polycyclic aromatic compounds 5a and 6a, along with pyranopyran 2a and 1,2?naphthoquinone?1?methide dimers 3a and 4a. Isolation of 3a and 4a is indicative of the generation of 1,2?naphthoquinone?1?methide from bromide 1a under mild basic conditions. A mechanism involving Michael addition of tetrachlorocatechol to 1,2?naphthoquinone?1?methide has been proposed for the formation of diastereomeric compounds 5a and 6a. A similar reaction of bromide 1a with tetrabromocatechol also resulted in monobromo diastereomeric compounds 5b and 6b, along with 2a, 3a, and 4a, confirming the proposed mechanism. Changing the solvent to methyl ethyl ketone gave corresponding diastereomeric compounds 5c, 6c, and 6d, along with other products. Similarly, reaction of bromide 1b with tetrabromocatechol in acetone/methyl ethyl ketone afforded compounds 5d, 6e, 5e, 6f, and 6g, along with pyranopyran 2b and quinone?methide dimers 3b and 4b. The structures assigned to these compounds, based on detailed spectral analyses, are discussed in Section II.

Section III deals with the reaction of 2?bromomethyl?1?(2??tetrahydropyranyloxy)benzene (7a) with tetrachlorocatechol in acetone in the presence of potassium carbonate. The reaction produced diastereomeric compounds 10a and 11a, along with two less?polar tetrachlorocatechol ethers 8a and 9a. The generality of the reaction was demonstrated by reacting bromides 7a–c with tetrachlorocatechol/tetrabromocatechol in acetone or methyl ethyl ketone, giving analogous compounds 8b–f, 9b–f, 10b–g, and 11b–h. A suitable mechanism has been proposed for the formation of tetrahalocatechol ethers 8 and 9, and supporting experimental evidence is presented.

In Section IV, the in situ generation of 5,6?quinolinedione?5?methide and 6?methylene?4?(2?pyridyl)cyclohexa?2,4?dien?1?one from 5?bromomethyl?6?acetoxyquinoline (12) and 4?(2?pyridyl)?2?bromomethyl phenylacetate (13), respectively, under mild basic conditions is described. The reaction afforded polycyclic aromatic compounds 5f, 6h, 10h, and 11i in the presence of tetrachlorocatechol in acetone. Section V contains the experimental details for all the above work.

Chapter II – Photoisomerization of Spironaphthalenones Chapter II deals with a study of the photoisomerization of various substituted spironaphthalenones of types 14 and 15, and is divided into two sections. Section I begins with a brief introduction to the photochemistry of 2,4?cyclohexadienones, followed by a discussion of the results obtained in the photoreactions of spironaphthalenones. Spironaphthalenones 14a–g and 15a–g were subjected to photoirradiation using a pyrex filter. The irradiation experiments showed that all the ??spironaphthalenones undergo isomerization to their corresponding ??isomers upon irradiation, whereas among the ??spironaphthalenones studied, only 6?methoxy? and 6?bromo???spironaphthalenone (15f and 15g) undergo isomerization to the ??isomer. A plausible mechanism involving the intermediacy of a spirocyclohexadienone 16, formed by initial ?–C–O bond cleavage of the spironaphthalenones, has been proposed for the observed photoisomerization. The experimental details are given in Section II.