Dehalogenation tosylate

Cycloheptanes.— The C-1—C-2 bond in -y-thujaplicin is essentially single, Co"-/3-thujaplicin-amine complexes have been described, and thermodynamic data on the U -/3-thujaplicin complex have been calculated. The biomimetic cyclization of the silyl enol ether (191) to karahanaenone (192), using methyl-aluminium bis(trifluoroacetate) is almost quantitative (192) is also synthesized by thermolysis followed by desilylation of the silyl enol ether (193) which is readily available from l-bromo-2-methyl-2-vinylcyclopropane and isobutyraldehyde. Dehalogenation of 3-bromo-l-iodo-3-methylbutan-2-one with Zn-Cu couple on alumina in the presence of isoprene yields (192) and minor amounts of the isomers (194) and (195) however, dehalogenation with Fc2(CO)9 favours (195). Acetolysis of karahanaenol tosylate yields anticipated p-menthane derivatives and no filifolene. ... 

The classic ionic methods for the removal of an unwanted hydroxyl group are summarized in detail in the book by Larock [6]. One method involves—for primary and unhindered secondary alcohols—the synthesis of the corresponding mesylates or tosylates. These compounds are prepared readily and then transformed into the corresponding deoxy compounds by reduction [7], Alternatively, introduction of a thiolate or halogen by a nucleophilic reaction can also be used. These compounds can then be readily desulfurized or dehalogenated (Scheme 1). Tertiary alcohols present no problem either, because a... 

Halolactamization.3 Unsaturated amides generally form lactones when cyclized by Br2 or I2. However the unsaturated N-tosyl amide I, when treated with Br2 in the presence of N a HC03, forms the bromo N-tosyl /J-lactam 2 (67%yield). Dehalogenation of 2 is best effected with Bu3SnH. 

Many deoxy-sugars are synthesized by conventional methods of dehalogenation or reduction of thiocarbonates tosylates, mesylates etc. One notable exception, involving dimerization of phenylselenyl 2-deoxy-D-glucopyranose in a concentrated solution and in the absence of trapping agents is worthy of mention as it is related to the chemical character of the free radical intermediate. The approach proceeds by the formation of the axial-axial coupling product as the major one and axial-equatorial as the minor product. 

An alternative annulation reaction results in the formation of [l.l.ljpropellane 25." - Hy-droxymethylation of Moore s hydrocarbon 24, followed by conversion to a bromo alcohol by treatment with butyllithium and tosyl bromide, then conversion into a chlorobromo compound with carbon tetrachloride/triphenylphosphane, which was dehalogenated with methyllithium, gave the [l.l.ljpropellane derivative 25. 

The first synthesis was developed in Tatsuta s laboratories and is outlined in Scheme 2.21. The C-11 to C-15 fragment, containing one chiral center, was constructed from the 2-deoxy-D-arabino-pyranoside 236. Thus treatment of 236 with sulfuryl chloride followed by Nal in methanol gave the dichloropyranoside 237 (41%). Conversion to the pyranose 238 was accomplished in 78% overall yield by acylation, reductive dehalogenation, and hydrolysis. Tosylation then gave the unstable gly cal 239, which was immediately oxymercurated and hydrolized to give enal 240 in 51% yield. 

A different approach to methyl ohvosides (24), starting from L-arabinose, is much more laborious and involves chain extension through the Henry reaction (Scheme 2). The 2-deoxy function is introduced by elimination/hydro-genation and 6-deoxygenation is achieved either via tosylation/LAH reduction or iodination/Raney nickel reductive dehalogenation of the primary hydroxyl group [52]. 

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