Tosylates, acetyl, formation

Durst and Sharma [86] have reported the stereospecific synthesis of 3-spiro-epoxyazetidin-2-ones 55 (Scheme 14). The oxidation of the diastereoisomers of compound 52 with PCC provided a single acetyl compound 3-acetyl-3-benzyloxy-azetidin-2-one 53. Nonchelation controlled L-Selectride reduction of 53 gave the isomerically pure 3-hydroxyethylazetidin-2-one as the sole reduced product, which was further converted to tosylate 54 using NaH/tosylimidazole. The debenzylation-oxirane formation sequence was conveniently performed as a single pot operation with ammonium formate, 5% Pd/C in refluxing methanol as the hydrogen transfer reagent combination. 

Some examples are known where the formation of alternative, anhydro-ring structures is possible, and the deformation of the molecule required in the alternative reactions is approximately the same, so that entropy changes may be largely neglected. In such cases the ethylene oxide ring is not favored. The action of bases on methyl 3,4-di-0-acetyl-2,6-di-0-tosyl-/J-D-glucopyranoside and on methyl 2,6-di-0-mesyl- -D-galactopyranoside affords predominantly the 3,6-anhydro-2-0-sulfonyl derivative in each case. 

Related to the Heck reaction is the Larock annulation of internal alkynes, which is a general route to heterocyclic and carbocyclic systems. Especially attractive is the construction of the pharmaceutically important indole ring system via palladium-catalyzed coupling of 2-iodo-aniline and the corresponding V-methyl, acetyl, and tosyl derivatives with a wide variety of internal alkynes. The catalytic process appears to involve arylpalladium formation, regioselective addition to the carbon-carbon triple bond, and subsequent intramolecular palladium displacement.- ... 

Acyloxonium ions (Section 7.3) are stabilized by heavy anions. The formation of acyloxonium ions is also assumed, this time as non-isolated intermediates, in the solvolysis of some tosylates. In reaction (7,12) with sodium acetate, the introduction of an acetyl group at position 6 and migration of benzoate to position 5 testify to the presence of a benzoxonium intermediate. Likewise, solvolysis of sulfonates presenting a frani -vicinal acetamido group involves an oxazolinium cation 7.28, the analogue of an acyloxonium. 

The reaction between tri-O-acetyl-D-glucal 7 and chlorosulfonyl isocyanate has been studied in the past, but neither formation of a cycloadduct nor of a rearranged product has been observed. Isocyanate acted only as acid catalyst causing decomposition of sugar material. On the other hand, [2+2]cycloaddition of active isocyanates to dihydro-2H-pyran and to its derivatives has been widely investigated, under a variety of conditions. The reaction of tosyl isocyanate with dihydro-2H-pyran 1 at low temperature (0 ) led to the formation of bicyclic p-lactam 2. Elevation of the cyclization temperature resulted in the rearrangement of the four-membered ring to the open-chain amide 3 (Scheme 4). 

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