Rearrangement acyloxy acetates

The naturally occurring cryptand aplasmomycin (190) is one of a unique family of ionophore antibiotics, and several syntheses of the total structure, half structure and various abbreviated segments have been published. White et al. have now described a new approach to (190) which features a novel ring contraction based on the rearrangement of a-(acyloxy) acetates originally described by Chan et al. Thus, the key intermediate (191) was treated successively with lithium diisopropylamide and trimethylsilyl triflate leading to (192) which was ultimately converted to aplasmomycin. In Corey s recent... 

Ion pair collapse in the acyloxy migration is so rapid as to preclude nucleophilic trapping of the contact ion pair even by intramolecular nucleophiles, which essentially precludes the use of acetates as leaving groups in tandem rearrangement reactions of the types discussed below [111, 127]. 

OL-Acyloxy aldehydes,2 The N-f-butylnitrones (2) of aldehydes react with acid chlorides in the presence of triethylamine to form a-acyloxy imines (3) via a [3.3] sigmatropic rearrangement. The imines are hydrolyzed by buffered aqueous acetic acid to a-acyloxy aldehydes (4). 

Dialkyl [(2-acyloxy)alkyl]phosphonates, neat or in solution, undergo a fragmentation-rearrangement process when pyrolysed at ca 220-240 °C the overall reaction, represented in equation 7, results in moderate to high yields of alkenes and of acetic esters . A similar decomposition occurs under ultraviolet irradiation... 

By the action of acetic anhydride in an organic solvent the sulfoxide of (377 R = H) rearranged to the 2-acyloxy derivative (377 R = OAc) <90EUP404525>, while the sulfoxide of the thia-zinoquinazoline (379) yielded the 1-acyloxy derivative <87JPR321>. The former reaction in the presence of excess triethylamine resulted in the hydroxy derivative (377 R = OH) <90EUP404525>. 

In a series of papers, Kucklander has provided additional evidence for the Nenitzescu-Allen mechanism, in particular theD->-E- F- G transformation [18-20]. Eor example, isolated carbinolamine 35 gives byproducts 36-38 in addition to indole 39 (Scheme 7, equation 1) [18]. The formation of 36 presumably involves reaction of iminium ion 40 with acetate and rearrangement of 41 to 36 as shown (Scheme 7, eqnation 2) [20], Kucklander and Hiihnermann found that benzoquinone reacts with p-arylaminocrotonates in propionic acid to afford 6-hydroxyindoles 42 (10%-30%) (equation 3), along with the byproducts 5-acyloxy-4-hydroxyindoles (20%-35%) and arylaminobenzofurans (10%-15%) [21]. 

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