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. 

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. 

In very dilute base or, preferably, in the presence of weak bases, the homoannular enolate (16) is formed which can be adsorbed in either a cis (17) or a trans (18) manner. In this case the presence of a methyl group results in a slight favoring of trans adsorption, thus leading to the formation of a slight excess of the m-product as is observed on hydrogeneration of A" -3-keto steroids in the presence of triethylamine. ... 

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. 

Wally et al.115 report a homoannularly bridged hydroxyamino ferrocene (+)-123 as an efficient catalyst for enantioselective ethylation of aromatic or aliphatic aldehydes. 

The shorter wavelength bands (200-300 mp) could be similar (but less forbidden) charge-transfer transitions or homoannular ring-localized transitions. For instance, Nesmeyanov10 concludes, from an inspection of oscillator strengths, that the strong 200 mp band is a -n -s- tt transition in the ring. 

The major product is the heteroannular disubstituted derivative, l,l -diacetyl-ferrocene (I), while a very small amount of a homoannular isomer, 1,2-diacetylfer-rocene (II), is also obtained. The first acetyl group therefore appears to deactivate the substituted ring toward further electrophilic substitution, and the second acetyl group preferentially enters the opposite ring. 

Diarene metals generally decompose under conditions of normal electrophilic substitution, but dibenzene chromium can be metalated with amyl sodium in alkanes, and subsequent reaction with carbon dioxide and dimethyl sulfate results in a complex mixture of mono-, di-, and poly-substituted dibenzene chromium methyl esters. It is interesting that these polysubstituted compounds are homoannular in contrast to the corresponding ferrocene compounds. In view of the scanty evidence and ease of oxidation, it is impossible to conclude whether the diarene metals are less aromatic than the dicyclopentadienyl metals as predicted by simple theory (Sec. III.A). 

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... 

In other examples, compounds in which a metal atom is already coordinated in a molecule can be used as a comonomer in an addition polymerization. Two examples involve the ferrocenes discussed in Chapter 6. The vinyl ferrocene molecule is shown in 7.14, and a similar vinyl manganese complex in 7.15.30 An alternative approach involves condensation polymerization. For example, if the R group in the ferrocene unit shown in 7.16 contains a hydroxyl group, it can be copolymerized with a diacid chloride. If it is an acid chloride, it can be copolymerized with a diamine. (This type of polymer is called a heteroannular chain if only one of the rings in the repeat unit is in the backbone, the polymer is called homoannular.)7 Similarly, the titanium complex shown in 7.17 is copolymerized with diacids or diols.30 Numerous other examples involving ferrocenes are discussed in Chapter 6. 

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