Sigmatropic rearrangements thermolysis

The use of chlorodiphenylphosphine to induce a [2,3]-sigmatropic rearrangement of enediynyl propargylic alcohols is one of the first synthetic methods adopted for the preparation of enyne-allenes. For instance, treatment of 108 with chlorodiphenylphosphine and triethylamine at -78 to 0°C afforded the enyne-allenylphosphine oxide 109 in 63% isolated yield (Scheme 20.23) [9]. Thermolysis of 109 at 37 °C in the presence of 1,4-CHD generated the biradical 110, leading to 111 and combina-... 

Thermolysis of 164 bearing a pendent olefin at 150 °C also allows the aryl radical in 167 to be captured in a 5-exo fashion, leading to 165 as a 1 1 mixture of diastereo-mers (Scheme 20.33). The corresponding Lewis acid-catalyzed [3,3]-sigmatropic rearrangement promoted by AgBF4 was facile at room temperature, making it possible for 166 to be isolated. Thermolysis of 166 at 75 °C afforded 165 in 80% yield. 

On thermolysis, appropriately substituted A-allyl-A-silyloxy enamines 19 undergo smooth [3,3]-sigmatropic rearrangements to the corresponding A-silyloxy imino ethers laP (equation 5). Two stereogenic centers are created but no reference to chiral induction is referred. High diastereoselectivity was observed and short reaction times favoured the syn A-silyloxy imino ether diastereomers. 

Pratasine. The utilisation of the cyclic hydroxamic acid 230 as a late precursor of pratosine has also been described [62] (Scheme 38). The preparation of 230 was achieved in excellent yield by photocyclisation of the borate ester 231 itself obtained from hydroxamic acid 232. Aqueous hydrolysis of the cyclic borate furnished 230, which underwent a smooth Michael addition to methyl propiolate to form the O-vinyl ester 233. The latter, on thermolysis in wet dimethylsulfoxide provided, via a 3,3-sigmatropic rearrangement, pratosine (204, 17% yield) and methyl pratosine-4-carboxylate (234,35% yield). 

Similarly, the photolysis of unsaturated a-glycoside 76 leads to the vinylglycoside 77 in low yields. This on thermolysis undergoes a 3.3 sigmatropic rearrangement to yield the C-branched sugar 78 (Scheme 40) [74]. 

An equilibrium of prop-2-yl selenocyanates (84) and allenes (85) was obtained on gas-phase thermolysis (0.1-0.01 Torr 350-400°C) of 84. This isomerization by [3,3]sigmatropic rearrangements provided access to the highly reactive allenyl isoselenocyanates (85). The isoselenocyanates (85) can only be handled in solution due to their pronounced tendency to polymerize. Although these are less reactive with nucleophiles than allenyl isothiocyanates, treatment with nucleophiles afforded selenazoles in moderate yield. [95AG(E)1627]... 

A tandem one-pot elimination-intramolecular Diels-Alder reaction occurs when the mesylate of 4-homoallylic azetidinone having a orc-alkene or alkyne substituent is heated in a sealed tube in the presence of an equimolecular quantity of l,8-diazabicyclo[5.4.0]undec-7-ene (DBU). The method has been used to produce derivatives of oxace-pham. In a similar way, the 3,4-disubstituted azetidinone mesylate 473 afforded an 88% yield of 474. The method can be further elaborated through the introduction of a novel [3,3]-sigmatropic rearrangement of a-allenic mesylates thus, 475 yielded 476 on thermolysis C1999TL1015, 2000JOC3310, 2005EJ098>. 

The highly substituted cir-divinylcyclopropanes (20) and (21) do not undergo sigmatropic rearrangement at all. Apparently, the highly sterically congested nature of the transition states (F) precludes this possibility. Thermolysis of (20) and (21) at 170-180 C produces only equilibrium mixtures of these substances and the corresponding trans isomers (22) and (23), respectively (Scheme 3). ... 

It was found that just contact with neutral alumina at room temperature in benzene or chloroform suffices to promote quantitative isomerization of compound I to the frani-dihydrofuran derivative II only, with retention of the configuration of the aryl and methyl substituents of cyclopropane I. This highly unusual result was underscored by the formation of both II and III during the parallel thermolysis experiment, a result that follows well established lines. Thus, dihydrofurans II and III are very probably the result of a 1,3-sigmatropic rearrangement formally similar to that described in Problem 14. 

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