Homoannular dienes

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... 

Only the heteroannular diene 523 is formed by treatment of both a- and, 3-allylic carbonates 522 and 524 in a hydrindan. system with a Pd catalyst. No homoannular diene is formed. 

Heteroannular and acyclic dienes usually display molar absorptivities in the 8000 to 20 000 range, whereas homoannular dienes are in the 5000 to 8000 range. 

High vacuum sublimation of the 3-monobenzoate of (32) or tosylchloride dehydration gives 3/ -benzoyloxy-7-methylenecholest-5-ene (33). The configuration of (32) is probably as shown since 7a-hydroxy steroids dehydrate readily to yield homoannular dienes. In an analogous reaction ethyl and iso-butyl Grignard reagents give alkylidene compounds as the only isolated products. 

Positively activated olefins have also been condensed with dienamines derived from aldehydes 321,330,347,348) and ketones. Of special interest is the formation of bridged systems from homoannular dienes (229-231) which has been applied to the isoquinuclidine system of the iboga alkaloids (137-140,349). 

However, an important difficulty is still present in fact, while for heteroannular cisoid dienes the allylic axial contributions are opposite in sign to the intrinsic, as depicted in Figure 6, in the case of the homoannular cisoid compounds, the two contributions have the same sign, as pointed out already by Burgstahler. 

Enolization of a,/J-unsaturated ketones, e.g. 19, under strong acid conditions leads to a mixture of homoannular and heteroannular A2,4- and A4,6-dienes (e.g. 20 and 21 see equation 6)42. 

Parent heteroannular diene Parent homoannular diene Add for each substituent ... 

A Transoid 1,3-Diene. In contrast to the considerable number of photochemical rearrangements of homoannular dienes only one photochemical rearrangement of a transoid heteroannular diene has been reported. Irradiation of 3,5-cholestadiene (Formula 400) in ethanol gives the pentacyclic ether (Formula 401) (176). 

Structure (XII) with the attachment of the lactone bridge reversed can be ruled out for the following reason. Gibberellic acid decomposes slowly in aqueous solution to give gibberellenic acid first described by Gerzon, Bird, and Woolf (16) who suggested the homoannular diene structure (XV). We consider that the ultraviolet absorption (Amax 253 m/ e 22,400) is more in accord with the hetero-annular structure (XIV) and this is confirmed (25) by ultraviolet absorption (Amax 309 mfi e 16,500) of the derived dienone (XVII) which is decisive [cf. the ultraviolet absorption (Araax 310 m/ e 3900) of model cyclohexadienone (XVIII)]. 

In formulating the rules (Table 3.8) for cyclic dienes, these are classified as either heteroannular or homoannular, e.g. 

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... 

According to Bergmann and Hirschmann,198 only homoannular dienes of the steroids are expected to give endoperoxides. The for-... 

Estrones. An A —> BCD approach to estrones involves a cobalt catalyzed intramolecular 2 + 2 -y 2] cycloaddition of the enediyne 1 to form the B, C. and D rings in one step. The high stereoselectivity suggests that the cyclization involves a Dicls-Alder-type reaction of the vinyl group with a cobaltacyclopentadiene formed by coupling of the two alkyne units. The homoannular diene obtained on demetalation is isomcrized easily to the diene 3. 

Naturally Occurring Substances.—The hydrocarbons from the oil of Juniperus sabina berries contain the homoannular abieta-8,9 13,14-diene, palustradiene, which readily forms an endoperoxide. The structure of ibozol (64), which was obtained from Iboza riparia (Labiatae), was assigned by examination of its C n.m.r. spectrum. A methylenetanshinquinone (65) has been described" as a... 

Undoubtedly one of the major synthetic achievements of the year has been the stereospecific total synthesis of gibberellic.acid (202). The tricyclic compound (200) formed a key intermediate and this was converted into the homoannular diene (201) and thence into gibberellic acid. Another strategy for gibberellin... 

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

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