1.3- and 3,3-sigmatropic shifts

The latter reacts with a second DMAD molecule to give cycloadduct (726). The reaction proceeds via opening of the aziridine ring to give compound (725). The cyclo-adduct (726) undergoes a 1,3-sigmatropic shift and aromatization to give compound (723) (Scheme 2.302) (816). 

A variant on the traditional method is to use a phenylhydroxylamine instead of a phenylhydrazine. For example, A-(benzyloxycarbonyl)-phenylhydroxylamine can be reacted with methyl propynoate to give an adduct that spontaneously undergoes a [3,3]-sigmatropic shift and eventually yields l-(benzyloxycarbonyl)-3-(methoxycarbonyl)indole (Scheme 7.15). 

Ethyl cyanoacetate in the presence of piperidine may also be used as the carbanionic component in reactions with salicylaldehyde. The initial Knoevenagel condensation is followed by a [l,7]-sigmatropic shift and cyclization to the 2-iminochromene derivative which adds another cyanoacetate molecule (67AP1). 

Cyanopyridine (pAfa 1.45) yields a trace of tetramethyl 9-cyano-9a77-quinolizine-l,2,3,4-tetracarboxylate (cf. 5) with DMAD the only product isolable from the 4-cyano isomer was the trimethyl 7-cyanoin-dolizine-l,2,3-tricarboxylate (cf. 35), whereas 2-methoxypyridine yielded 9 and ll.238 Cyclization in one direction to give 8 followed by a [1,5] sigmatropic shift, and pericyclic ring opening would lead to 9. The alternative cyclization to 10, followed by ionization and recombination, or a [1,5] shift, gives a second product 11, which has also been obtained from the quinolizinium salt 12. 

On refluxing in benzene with excess pyridine, the monocycloadduct of diphenylnitrile imine and pyridine smoothly underwent a [l,5]-sigmatropic shift and subsequent electrocyclic ring opening to afford 1,2,4-triazole derivative (44) (Scheme 33) <95H(40)515>. 

Thiocarbonyl oxides are a subject of active investigation. The natural occurrence of sulfines and related compounds in plants of the genus Allium (onion, garlic, etc.) is included in a superb and extensive review by Block [91]. Two detailed papers [92, 93] report the isolation of zwiebelanes from onions and their chemical synthesis involving intermediate sulfines produced by oxidation of di-l-propenyl disulfide, subsequent sulfoxide accelerated [3.3] sigmatropic shift and the [2+2] cycloaddition of the C=S and C=S=0 moieties. A further article [94] provides a great deal of information on the mechanism of formation of (Z)-propanethial S-oxide, the lachrymatory factor of the onion, as well as its chemical synthesis and reactions. Techniques of analysis of the volatiles of onions have been further improved [95]. 

In certain systems — especially in those carrying more than one donor or acceptor group — a ring expansion to five-membered heterocycles 11 occurs at relatively low temperatures. This process can be classified as a 1,3-sigmatropic shift and is most conveniently explained by the intermediacy of the 1,3-zwitterion 10. 

Two pericyclic reactions a sigmatropic shift and a cycloaddition in one reaction scheme. Suggested solution... 

In contrast to tetracyclo[4.3.0.0 .0 ]non-8-eneJ the corresponding 5-benzhydrylidene and 5-isopropylidene derivatives 15 (R = Ph, Me) do not undergo photochemical [27t + 2o-] cycloaddition but a formal [ 2 + 2 + 23] (retro-ene) reaction to furnish the substituted 5-methylenetricyclo[4.3.0.0 ]nona-3,7-dienes 16. The isopropylidene derivatives 17 also rearrange via a 1,3-sigmatropic shift and retro-Diels-Alder reaction to give the bridged cyclooc-tatrienes 18. 

The Carroll reaction uses an acetoacetate ester (54), made by ester exchange or with diketene (Chapter 33), to give enol (55) which can do the [3,3] sigmatropic shift and give keto acid (56) which decarboxylates under the reaction conditions. The synthesis of (53) is standard acetylene chemistry (Chapter 16). 

When enantiomerically pure 24 is used, the product has an e.e. >95% and the configuration shown above. This result establishes that chirality is maintained throughout the course of the reaction. This stereospecificity is a general feature of [3,3]-sigmatropic shifts and has made them valuable reactions in enantiospecific syntheses. ... 

Photolysis of both acyclic and cyclic 1,5-dienes leads to both 1,3-sigmatropic shifts and formation of allylcyclopropanes by 1,2-shift followed by cyclization. The product mixture from 1,5-cyclooctadiene illustrates these reaction patterns. 

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