3.3- Sigmatropic rearrangement allylic 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. 

The premier example of this process in an ylide transformation designed for [2,3]-sigmatropic rearrangement is reported in Eq. 15 [107]. The threo product 47 is dominant with the use of the chiral Rh2(MEOX)4 catalysts but is the minor product with Rh2(OAc)4. That this process occurs through the metal-stabilized ylide rather than a chiral free ylide was shown from asymmetric induction using allyl iodide and ethyl diazoacetate [107]. Somewhat lower enantioselectivities have been observed in other systems [108]. 

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 investigators 3 jj... 

Notice that the final result is a 1,3 charge affinity inversion Umpolung) of an allylic derivative via a FGI of a functional group of type E by a group of type A, followed by a [2,3]-sigmatropic rearrangement. If the intermediate allyl anion reacts with a carbonyl compound as the electrophile the result is then a 1,4-D system, such as ... 

In the solution proposed by Evans [25], however, the unsaturated ketone is condensed with the anion of an alkyl allyl ether, followed by a [3,3]-sigmatropic oxy-Cope type rearrangement (Scheme 5.22). In the retrosynthetic sense this means a [3,3]-sigmatropic rearrangement of the bis-enol form of the 1,6-dicarbonyl system. 

Uemura and co-workers developed an interesting and unique catalytic system, in which the Rh(ii) intermediate is generated by Rh2(OAc)4-catalyzed reaction of conjugated ene-yne-carbonyl compounds (Equation (16)). The Rh(ii)-carbene is trapped by allyl sulfide to give [2,3]-sigmatropic rearrangement product 121 in good yields." ... 

Note that the pi-system of the allyl group is readily aligned over the aromatic ring, providing a 1,5-diene motif for the sigmatropic rearrangement. 

Some ester- or aldehyde-substituted bicyclo[2.2.2]oct-2-ene and bicyclo[2.2.1]hept-2-ene systems (25) have been shown to undergo nitrosation adjacent to the carbonyl group followed by [3,3]-sigmatropic rearrangement to give oxazine products (26).23 Stereodeflned monoprotected allylic 1,2-diol (27) and its diastereomer have been ( ) converted into stereodeflned propenylimidazolidinone (30) and its diastereomer by... 

For analysis of the photochemical reaction, the interaction of the hydrogen Is orbital with -tt3 of the allyl system is used. The interaction is bonding at both the migration origin and terminus, so the [ 1,3] sigmatropic rearrangement is photochemically allowed. 

The first step in Cr(VI) oxidations can take place to give a chromate ester (Chapter 24) but this intermediate has no proton to lose so it transfers the chromate to the other end of the allylic system where there is a proton. The chromate transfer can be drawn as a [3,3]-sigmatropic rearrangement. 

Monosubstitution or different substituents at C-l of the allylic moiety open up the possibility for formation of (E)- or (Z)-allylic alcohols in acyclic systems. The transition state with one substituent R at C-l in an equatorial position ( transoid transition state) seems to be about 1.5 keal/mol more stable than the cisoid transition state7. The sigmatropic rearrangement via the energetically preferred transition state yields (T)-alkenes, usually with a selectivity of greater than 95 %86,87. 

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