Rearrangements cyclic systems

In addition to the synthetic applications related to the stereoselective or stereospecific syntheses of various systems, especially natural products, described in the previous subsection, a number of general synthetic uses of the reversible [2,3]-sigmatropic rearrangement of allylic sulfoxides are presented below. Several investigators110-113 have employed the allylic sulfenate-to-sulfoxide equilibrium in combination with the syn elimination of the latter as a method for the synthesis of conjugated dienes. For example, Reich and coworkers110,111 have reported a detailed study on the conversion of allylic alcohols to 1,3-dienes by sequential sulfenate sulfoxide rearrangement and syn elimination of the sulfoxide. This method of mild and efficient 1,4-dehydration of allylic alcohols has also been shown to proceed with overall cis stereochemistry in cyclic systems, as illustrated by equation 25. The reaction of trans-46 proceeds almost instantaneously at room temperature, while that of the cis-alcohol is much slower. This method has been subsequently applied for the synthesis of several natural products, such as the stereoselective transformation of the allylic alcohol 48 into the sex pheromone of the Red Bollworm Moth (49)112 and the conversion of isocodeine (50) into 6-demethoxythebaine (51)113. 

A more detailed analysis of bystander substituent effects considers rearrangements in cyclic systems e.g., cyclohexylidene, 56.8... 

A similar process involving an all-carbon cyclic system has also been investigated [59]. Other examples involving the prototropic rearrangement of enediynes having an imino or a keto substituent at the propargylic position to form the corresponding enyne-allenes have also been observed [60, 61]. 

Shubin and colleagues have described a series of rearrangements of unsaturated cyclic systems which occur via cationoid intermediates. Protonation of the triene 275 with... 

Another application of this strategy is the construction of cyclic systems bearing 1,4-dissonant relationships. For example, the synthesis of the hasubanan alkaloid ring system 35. reported in 1972 by Evans [24], involves the Diels-Alder cycloaddition of a dienyl sulphoxide 32 with an endocyclic enamine 33, followed by a [2,3]-sigmatropic rearrangement of the resulting cycloadduct 34 (Scheme 5.21). 

Up to now, we have seen how the "heuristic principles" can be applied to both linear and cyclic systems which we could call "normal systems". When some unusual structural features -such as quaternary carbon atoms, medium-sized rings or bridged systems- are found in the molecular structure under consideration, one may assume that they are derived either from a rearrangement or from an internal fragmentation [1]. 

The study of the photochemical rearrangements of a,/3-epoxyketones has been extended to include cyclic systems and, in particular,... 

Dienones, which because of the electron deficiency induced at the carbonyl carbon may be regarded as analogs of pentadienyl cations, are known to close photochemically.103 These closures occur readily in cyclic systems where the geometry requires the disrotatory mode, but the proposed intermediate product, formally a 1,3 diradical, is ordinarily stabilized through rearrangement.104 Woodward and co-workers have demonstrated the predicted photochemical... 

Conversion of the trimer (80) to the seven-membered ring system (81) occurs readily with cesium fluoride [3,75], whereas in the presence of TAS fluoride, the di-anion (82) is trapped. These observations lead to the most likely mechanism for rearrangement as that in Scheme 37 [3]. The cyclisation step shown in Scheme 37 is made more easily accepted by the fact that the diene (83) (Scheme 38), undergoes rapid rearrangement in the presence of fluoride ion, giving the cyclic system (86) [77]. The ready cyclisation of a crowded anion (84) to give what appears to be a sterically unfavourable intermediate (85) would not be easily predictable ... 

The selection rules for sigmatropic rearrangements were derived for open-chain systems. A priori, it is not obvious that they also apply to cyclic systems. Using the PMO method, show that the following sigmatropic migrations21 are allowed ... 

In recent years, the Stevens rearrangement applied to cyclic systems has resulted in the formation of a variety of novel heterocycles. For example, benzylammonium salt 7 was found to undergo a facile [1,21-benzyl shift in aqueous sodium hydroxide at 5 °C to form the tetrahy-droisoquinoline derivative 8.11... 

Strained cyclic systems such as cyclopropane derivatives proved sensitive to silver ions. Indeed, silver usually induced ring opening of cyclopropanes, leading to argento cationic intermediates, which evolved further depending on substituents. Most, if not all, of these ring rearrangements resulted from initial formation of a a complex of the cyclopropane with silver ion. 

The retro 1,4-Brook rearrangement in cyclic systems was unfavorable when the pertinent siloxy groups were in an anti position to the carbanion centers (equations 163 and 164). When a 2 1 mixture of cis- and trans-251 was treated with t-BuLi at —78°C and then warmed, and stood at +20 °C for 1 h followed by hydrolysis, trans-251 was completely transformed to trans-259, while 85% of cis-257 was converted to cis-258, indicating that the 1,4-silyl migration was confined to cis-257409. In some other cyclic systems, a retro 1,4-Brook rearrangement was observed even in the anti isomers410. 

In a cyclic system, the stereochemistry of the product of a [2,3]-Wittig rearrangement may be ensured by the rearrangement s suprafacial nature. [2,3]-Wittig rearrangement of lithiomethyl ethers derived from stannanes provides a valuable method for stereospecific... 

Although a chairlike transition state is favored for the Claisen rearrangement reactions of acyclic substrates, this is not always the case with cyclic systems. For example, Bartlett and Ireland independently studied the rearrangement reactions of cyclohexenyl silylketeneacetals and found that there was competition between the chairlike and boatlike transition states11 (Scheme l.VII). Clearly, the -isomer IE gives 8a via a chairlike transition state, whereas the Z-isomer 7Z affords the same product (8a) via a boatlike transition state. 

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