STUDIES DIRECTED TOWARD THE TOTAL SYNTHESIS OF CERORUBENIC ACID-III .3. A CONVERGENT ENANTIOSELECTIVE APPROACH INVOLVING NEW ARRANGEMENTS FOR THE ACTUATION OF RING-D CYCLIZATION

A possible enantiocontrolled route to cerorubenic acid-III (1) is desc ribed herein. The central elements of the approach take advantage of t he ready availability of both optical an of citronellol and of the tri cyclic ketone 8. Although the absolute configuration of 1 is not known , its relative stereochemistry has been established by X-ray crystallo graphy. On this basis, it is possible to match the reaction partners w ith the long-range view of establishing experimentally the actual conf iguration of 1. Assembly of the two structural halves was accomplished via a vinylstannane intermediate whose construction required utilizat ion of an intramolecular S(N)2 displacement in order to overcome the w eakly nucleophilic nature of a stannyl-substituted ester enolate. The coupling reaction leading to 38 and 39 proved highly stereoselective a s before, thereby setting the stage for conversion to D-seco derivativ es of 1 via anionic oxy-Cope rearrangement. Although this phase of the strategy had previously been worked out on simpler systems, 38, 39, a nd analogs therof isomerized with concomitant alpha-hydroxylation of t he intermediate enolates. These conversions proceeded smoothly and eff iciently. Although conditions were found that curtailed this adventiti ous oxygenation, the conversions proceeded so slowly that degradation was now competitive with the formation of 41 and 43.