Diene, homoannular

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. 

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). 

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)]. 

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. 

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. 

Basic homoannular diene Exocyclic double bonds (2 x 5) Substituents R (4 x 5)... 

Homoannular dienes e.g. i) readily add excited oxygen in a reaction which is the photochemical equivalent of the Diels-Alder diene-addition reaction fyjJ. The resulting epidioxides e.g. 2) are sufficiently stable for isolation/although they are fairly reactive [72]. 

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. 

A dramatic example of prevalence of y-alkylation over a-alkylation is provided by cyanoethylation of 3)S-acetoxy-5a-pregn-17-en-21-al (186) with acrylonitrile in benzene (see Part II, Chapter 1, refs. 213—216). The signs of the Cotton effects exhibited by the isomeric homoannular dienes (187) produced in this reaction violate the helicity rule for skewed dienes. ... 

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