Homoannular conjugated diene

In steroid systems, the homoannular diene in ring A and the heteroannular diene in AB rings are generated. The allylic 3a-carbonate 514 affords the homoannular conjugated diene 515 as a main product and a small amount of the heteroannular diene 516. On the other hand, the heteroannular conjugated diene 516 is obtained exclusively from 33-carbonates 517. The elimination reaction proceeds smoothly at room temperature. 

The reaction has been applied successfully to the synthesis of a precursor of provitamin D. 520, which has a homoannular conjugated diene in the B ring[340]. Treatment of the 7a-carbonate 518 with Pd cataJy.st at 40 C afforded the 5.7-diene 520 regioselcctively in good yield. No heteroannular diene 521... 

Hetero- or homoannular conjugated dienes are prepared in hydronaphthalene derivatives by the Pd-catalyzed regioselective elimination of allylic carbonates under mild conditions. The heteroannular conjugated diene 496 was obtained selectively by the Pd-catalyzed regioselective elimination of the -oriented allylic carbonate 494 under mild conditions via the stable intermediate 495. The homoan-... 

The base Amax value for the former is taken as 214 nm and for the latter as 253 nm. To these values are added increments according to the nature of the substituent present and other structural features as shown in the table. In those cases where homoannular and heteroannular chromophores coexist in conjugation, the base value for the homoannular contribution is used, to which is added the appropriate increment for extended conjugation. It should also be noted that homoannular dienes in ring systems other than the six-membered ring do not give a satisfactory correlation between calculated and experimentally determined Amax values. The rules however are adequate for acyclic or heteroannular... 

A homoannular diene is one in which the two double bonds are conjugated and are in a single ring. 

Braude s data for a cis-S-cis system. In the UV of 51, the absorption maxima are situated at 230 and 270 m u they are consistent with a cis-S-cis structure. In fact, investigation of models shows that the side chain lies in a different plane from that of the ring. The absorption at 270 m u corresponds to that of a disubstituted homoannular diene, the absorption at 230 mM to the S-m form of a conjugated ester. On the other hand, the lactonization of the crude product from irradiation of 49 provides evidence for the formation of the two acids, 52 and 53, since it enables us to separate a 5-lactone 54 and a 7-lactone 55. The latter is produced by lactonization of 53 and shows a maximum at 212 m u, which disappears after reduction. 

Each of the other two compounds exhibits an isolated tz bond. One of those compounds does exhibit a homoannular diene, which does add +39, but the compound above also has two n bonds in the same ring, so it also gets +39. The compound above will have the largest because it has the most extended conjugated system. 

Potassium tert-butoxide is a strong, stericaUy hindered base, and the starting material will react with two equivalents of this base to undergo two successive elimination (E2) reactions, producing a conjugated, homoannular diene, as shown. 

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