Elimination with bicyclo compounds

Based on his previous work on the catalytic double addition of diazo compounds to alkynes173 using Cp RuCl(COD),174 Dixneuf has developed an efficient one-step synthesis of alkenyl bicyclo[3.1.0]-hexane derivatives of type 163 from enyne precursors 162 (Scheme 43). The catalytic cycle starts with the formation of an Ru=CHR species. It then adds to an alkyne to form ruthenacyclobutene 166, which evolves into vinylcarbene 167. [2 + 2]-Cycloaddition of 167 gives ruthenacyclobutane 168. The novelty in this transformation is the subsequent reductive elimination to give 170 without leading to the formation of diene 169. This can be attributed to the steric hindrance of the CsMes-Ru group. 

The bicyclo[3.3.0]octene skeleton is of interest due to its potential use as an intermediate in the synthesis of cyclopentanoid natural and nonnatural compounds. The bicyclo[3.3.0]octene skeleton can be prepared by asymmetric elimination of the corresponding alcohols on treatment with a variety of chiral amines80, S1. Bicyclic trifluoromethanesulfonales 1 on treatment with (5)-Ar,Ar-dimethyl-l-phenylethylaminc give the corresponding bicyclo[3.3.0]octenes 2 with 15,5/ -configuration with up to 89% enantiomeric excess in high yield. 

This method of nitrogen elimination from tetrahydropyridazines is frequently used in the synthesis of polycyclic systems. The thermolysis, as well as photolysis, of 2,3-diazabicy-clo[2.2.1]hept-2-enes gave bicyclo[2.1.0]pentanes via intermediate formation of cyclopenta-1.3-diyl, from which bicyclic compounds with one three-membered and one four-membered segment are formed this type of reaction is discussed in Houben-Weyl, Vol. E17b, pp 1089-1108 and 1163-1166. 

Outstandingly different in character from the perfluoroalkyl derivatives described above are the lithium and magnesium compounds derived from highly fluorinated bicyclo-[2.2.1]heptanes [37, 38]. Some reactions of the lithio derivative are shown in Figure 10.8, illustrating that it can be utilised in the normal synthetic procedures with much less competition from elimination. 

An important field of application of the 1,3-elimination method concerns those compounds where oxiranes can be obtained with configurations opposite to those of the oxiranes resulting from the peracid method. Examples are the stereoselective epoxidation of a methylenecyclohexane derivative with NBA/KOH, the epoxida-tion of 3-alkylcyclopentene and 3,4-dialkylcyclopentene with NBS/KOH, and the /3-epoxidation of bicyclo[3.3.0]octene and bicyclo[4.3.0]nonene. The different stereoselectivities can be interpreted on the basis of the mechanisms of the... 

In their search for conformationally biased mimics of mannopyranosylamines, A. Vasella and co-workers planned to synthesize compounds that would inhibit p-mannosidases. In order to construct the bicyclo[3.1.0]hexane framework, a five-membered 0-silylated A/,A/-dimethyl-amino alcohol was synthesized. Oxidation of the tertiary amine with mCPBA yielded 83% of the A/-oxide, which was subsequently subjected to the Cope elimination to give 69% of the desired benzyl enol ether. Cyclopropanation of this enol ether gave rise to the highly functionalized bicyclic skeleton. 

The silyl-bridged four-membered rings are not A, B observed in these reactions with aminofluorosilyl-substituted rings. These Li salts react by one route to triply substituted products they also form bicyclo[4.2.0]octanes by an intramolecular reaction with LiF elimination and silyl group and methanide ion migration . Figure 4 shows the structure of compound... 

Compound (557) has been hydrolysed with the hope that, by analogy with the formation of trishomocyclopropenyl from cis-bicyclo[3,l,0]hex-3-yl tosylate, products from heptahomotropylium cation would be found. This was not realized and 97 % of the products arise by 1,2-elimination. In contrast to these negative results, a cyclopropyl substituent in arenes causes a large increase in... 

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