A Biomimetic Approach to Okilactomycin and Chrolactomycin and The Hexadehydro-Diels-Alder (HDDA) Reaction

Abstract

We hypothesized that spirotetronate (+)-okilactomycin (1) and (-)-chrolactomycin (2) are biogenetically derived from a common intermediate, (-)-okilactomycin D (3), which in turn arises via an intramolecular Diels-Alder (IMDA) reaction from the linear precursor 4. Guided by this hypothesis, we have achieved an efficient synthesis of okilactomycin D by a route featuring a substrate-controlled, diastereoselective intramolecular Diels-Alder (IMDA) reaction of an analogue of polyene 4. The assigned absolute configuration of (-)-3 was confirmed. Conversion of (-)-3 toward 1 and 2 has also been explored. <italic>ortho<italic>-Benzyne (1,2-didehydrobenzene, 7 or 7') is one of the oldest, most interesting, most useful and most well-studied of all reactive intermediates in chemistry. The multifaceted and efficient reactions of benzynes with suitable trapping reagents have long been employed in the service of synthetic chemistry to give products that are used as pharmaceuticals, agrochemicals, dyes, polymers, and other fine chemicals. An accidental observation made in this laboratory led us to establish an unorthodox yet general aryne-generating strategy--the hexadehydro-Diels-Alder (HDDA) reaction. This enabling transformation, which produces the highly reactive benzyne intermediate from the thermal [4+2] cycloisomerization of a 1,3-diyne (like 6) with a 'diynophile' (like 5) in the absence of any metals or reagents, has allowed us to uncover some unprecedented aryne reactivities.