Dienes heteroannular

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. 

Hence, assuming that the axial allylic contribution outweighs that due to the internal twist, the anomalous behaviour of s-cis heteroannular dienes is explained. 

Another, very notable, case where the two definitions are in conflict is that of het-eroannular cisoid dienes. As we have mentioned, this was just the class of molecules that stimulated the introduction of the AAR. Here, in order to have the correct results one should refer the chirality of the axial substituent to the individual double bonds (olefin-picture), as depicted in Figure 6 and in the upper parts of Figure 7(b) and (c). The case of heteroannular dienes is anyway peculiar, because in these compounds the chromophore is unusually distorted. This case is treated in the following section. 

We can collect here two classes of molecules, characterized by very large skew angles, between 40° and 90°. Such a situation is found in the first instance in heteroannular dienes. 

The two representations lead to the same result in the majority of the cases studied so far, where agreement with experiment is rather good and there are only few exceptions. The cases in which the two pictures are in conflict can be divided into two classes strongly versus normally distorted dienes (0 + 30°). The former is typified by the heteroannular dienes, which seem to obey the olefin-picture . The latter, much less well characterized, seems to be better interpreted in terms of the diene-picture . 

The heteroannular diene is thermodynamically more stable and the UV spectra of the two dienes differ, as suggested by Woodward and Fieser s rules5. 

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). 

A heteroannular diene is a conjugated system in which the two double bonds belong to two different rings. However, these double bonds are also exocyclic, one of them being exo-to ring A and the other exo-to ring B. 

Basic heteroannular diene Exocyclic double bonds (1 5)... 

Parent heteroannular diene Double bond extending the conjugation Alkyl substituent or ring residue Exocyclic location of double bond to any ring Groups OAc, OR... 

One of the major problems was the case of the cisoid heteroannular dienes , which present a CD band at about 240 nm with a sign opposite to that predicted on the basis of the diene helicity by means of the DR. 

Rapid access to salient heteroannular dienes by RCM of dienynes is represented by synthetic approaches to colchicine and the bicyclo[5.3.l]undeca-l,9-diene system. ... 

DMSO need not be the solvent, however, and cosolvents such as ethyl acetate often give better results. Many applications are possible due to the neutral oxidizing conditions and lack of side reactions, and a variety of functionalized substrates containing alcohol moieties have been oxidized. Tertiary alcohols are, of course, resistant to Moffatt oxidation and dehydration has been observed in such systems, 10 giving the alkene and regenerating DMSO.i O Some steroidal allylic alcohols dehydrate to a heteroannular diene under these condi-... 

Heteroannular diene (transoid or s-trans) More intense, e = 12,000-28,000 X shorter (234 nm)... 

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