Second‐Generation Total Synthesis of the Pigment Aurantricholone

Abstract

Our first total synthesis of aurantricholone established its benzotropolone core by the ring‐enlargement of a tetralone. Here we describe another total synthesis of aurantricholone. It reaches the benzotropolone core from a known olefin metathesis product via an equally known dibromide, both of which contain a ketoketal moiety. The next transformation ‐ step 9 overall ‐ engaged this motif in a β‐elimination of ROH rather than in a hydrolysis under the forcing acidic conditions indispensable in all prior benzotropolone preparations from such an intermediate. In step 10, the Csp2−Br bonds of the elimination product underwent two doubly Z‐selective Suzuki couplings with a boronylated O‐methyl 4‐methylidenetetronate. This gave penta(O‐methyl)aurantricholone. Its NMR shifts matched essentially those of a derivative of natural aurantricholone by Steglich et al. Three O−Me bonds were cleaved with BBr3/CH2Cl2 (step 11) and two O−Me bonds with LiBr/DMF (step 12). A 1 : 3 co‐crystal of aurantricholone and DMSO allowed for an X‐ray structure analysis.

Syntheses of 9‐Vinylated Benzotropolones by a Sequence of Acylation, Ring‐Closing Enyne Metathesis, and Oxidation

“…The RCM route to benzotropolones depicted in the upper half of Figure 1, like all others, [2a,b,d] employed the Grubbs‐II catalyst [3] and was applied to four type‐ 2 1,8‐dienes [2c] . This furnished the parent benzotropolone ( 4 , all R x =H) as well as three substituted benzotropolones 4 (up to three R x ≠H) [2c] .…”