Hypervalent N-triflylimino-λ(3)-bromane undergoes direct and regioselective α-C-H amination of ethers at room temperature under transition metal-free conditions. Kinetic results, substituent and deuterium isotope effects suggest an asynchronous concerted organonitrenoid transition state with some hydride transfer character, analogous to that for alkane C-H insertions.

Chual synthesis of the necic base, heliotridane has been achieved by employing a samarium diiodide-promoted regioselective carbon-carbon bond cleavage reaction of the y-halo ester (17), as a key step.Recently we developed samarium(I1) diiodide-promoted carbon-carbon bond cleavage reaction of y-halo carbonyl compoundsl and this novel fragmentation reaction was expected to have broad utility in the synthesis of natural products. In the continuation of a research program to exploit this synthetic strategy in the natural products synthesis,2,3 we planned the synthesis of pymolizidine alkaloids, and here report a chual synthesis of the necic base, heliotridane (I), in natural enantiomeric form, together with a stereoselective synthesis of pseudoheliotridane (2).F i t we investigated the synthesis of (-)-pseudo-heliotridane (2) starting from the ester (4),2 readily obtained from (-)-cawone, via the y-halo ester (3) utilizing a regioselective fragmentation reaction as a key step, as follows.Removal of triethylsilyl group of 4 in acetic acid-water-THF (3:l:l) afforded the alcohol (5) in quantitative yield, which on treatment with diphenylphosphoxyl azide according to the Lal's procedure4 provided the azide (6) with the desired stereochemistry in 77.2% yield. Reduction of 6 with magnesium powder in methanol5 for 3 h at 0°C proceeded smoothly to give the lactam (7) in 94.3% yield. In order to synthesize a bicyclic ring system, the olefinic lactam (7) was subjected to ozonolysis to afford the aldehyde (8) in quantitative yield. Wittig reaction of the aldehyde (8) using methoxymethyltriphenylphosphonium chloride and n-butyllithium provided the en01 ether (9) as a mixture of EIZ (ca. 1: 1 ratio) stereoisomers, which on treatment with aqueous acetic acid gave the cyclized product (10) as a single stereoisomer, in 24.3% yield from 8. Although the

No abstract

No abstract

The reductive regioselective bond-cleavage reaction of y-halo carbonyl compounds with samarium(tt) iodide is developed and its utilisation in the enantiospecific synthesis of (-)-oudemansin A is described.