Chromens and citrans derived from phloroacetophenone and phloroglucinaldehyde by citral condensation: regioselectivity, mechanism, and X-ray crystal structures

Abstract

The monochromen produced by low-temperature pyridine-catalysed condensation of phloroacetophenone with citral has structure (15), not the isomeric structure (17); on further heating of compound (15), or by direct reaction at higher temperature, two isomeric citrans (5) and (7)(8 : 1) are formed. Similar conclusions are reached for the chromen and citrans from phloroglucinaldehyde. The structure of the major acetyl citran (5) and of the major formyl citran (6) are established by X-ray studies of these strained species in which the aromatic ring assumes a shallow ‘boat’ form. During heating in pyridine, the chromens (15) and (16) isomerise to the chromens (17) and (18), which undergo more ready electrocyclisation giving the citrans (5) and (6), respectively; a mechanistic scheme is proposed.

Investigation of the acid-catalysed cyclisation of the chromen (15) shows that the citran (7) and the isopropenyl compound (38) are formed; no evidence to support a citran rearrangement in this reaction was found.