Reaction with oxiranes

Five-membered ring systems can be obtained from hetero-l,3-dienes on reaction with oxiranes and thiiranes. To avoid competition from a possible 1,4-addition, the nucleophilic attack of the terminal heteroatom of the diene has to be sterically or electronically hindered by incorporation of the heteroatom into... 

Several chapters deal with the synthesis of and the synthetic applications of organolithium compounds such as orthometallation, arene catalysed lithiation, addition to carbon-carbon double bonds, their reaction with oxiranes, and asymmetric deprotonation with lithium (-)-sparteine. We gratefully acknowledge the contributions of ah the authors of these chapters. 

Enantiomerically pure glycosyl stannanes gave, after tin-lithium exchange and reaction with oxiranes in the presence of boron trifluoride diethyl ether complex, alkylation products as diastereomers (1 1 -2 1 d.r.) with complete retention at the stereogenic center a to the oxygen atom41. 

An earlier experimental study involved the generation of benzylchlorocarbene, phenylchlorocarbene, methoxy-phenylcarbene, dimethylcarbene, cyclobutylidene, adamantylidene, and fluorenylidene by means of laser flash photolysis and their reactions with oxiranes and thiiranes <1998JA8681>. Absolute rate constants ranging from 10 to 10 ° M s at 22 °C were measured in acetonitrile and cyclohexane. Thermally generated dimethoxycarbene was found to be inefficient at abstracting oxygen or sulfur from an oxirane or a thiirane. 

All cationically polymerizable monomers can be potentially used in this process however, the main study has been focused so far on the most reactive oxirane and vinyl ethers [4], Alkoxysilane derivatives - the most common acid-sensitive monomers for the synthesis of siloxane materials through the use of sol-gel methods - were not used extensively. Only a few examples of their application in photo-activated cross-linking can be noted, mainly in co-reaction with oxirane sites [5]. Typically, alkoxysilanes are subjected to an acid- or base-catalyzed process involving hydrolysis of an =SiOR group and then condensation of the formed silanol with another molecule bearing an =SiOH or =SiOR function to give a siloxane linkage [6]. It was of interest to combine the properties of cross-linked silicone materials with the ones provided by sterically overloaded... 

Compared to glutardialdehyde mediated coupling, reactions with oxirane groups are slower. However, here again coupling times can have a considerable influence on stability. This is illustrated in Fig. 5 which shows the storage stability of penicillin G amidase immobilized on a polymethacrylate carrier in relation to the coupling time. 

Reaction with oxiranes (epoxides).2 Dimethylcopperlithium and diphenyl-copperlithium1 are more useful than other organometallic reagents for the nucleophilic ring opening of epoxides. Thus dimethylcopperlithium reacts with propylene oxide (equation 1) and 1,2-epoxybutane (equation 2) to give the expected secondary alcohols as the predominant products. 

In the reaction with oxirane, a mixture of 0.20 mol (8.8 g) of oxirane (Note 2) and 40 ml of Et20 is added over 15 min. The grey suspension is stirred for an additional half hour (Note 3), after which 15 g of powdered ammonium chloride is added in small portions (the dropping funnel and vent having been removed). The work-up is carried out as described above. After thorough removal of the solvent and other volatile components (first at 10-20 mmHg, subsequently at 1 mm or lower pressure), the pure alcohol is obtained in an almost quantitative yield. 

The reaction with oxirane, affording 0-F—C6H4—CH2CH2OH, can be carried out in a similar way and gives the alcohol in a reasonable (about 70%) yield. Oxirane is added in an excess of 30 %. In the absence of HMPT the reaction is very slow. Couplings with aldehydes and ketones or thiolation with elemental sulfur have to be carried out with the lithium compound, which can be prepared as indicated, or by addition of a solution of anhydrous LiBr in THF to the solution of the potassium compound. f-BuOLi present in the solution may give rise to aldol condensation of enolizable aldehydes and ketones, particularly at temperatures in the region of 0 °C. Therefore, the use of an excess of the aldehyde or ketone should be avoided. 

The Cg reactant used for step (vii) was synthesised from lithio-1-heptyne by reaction with oxirane foiiowed by catalytic reduction to the Z-alkene, non-3-enol, and conversion to the 1-iodide by reaction with chlorotrimethylsilane in acetonitrile containing lithium iodide. 

The allylic protons in cyclohexene are less acidic than those in aliphatic 1- and 2-alkenes. Prolonged ( 24h) treatment at 25 °C of a 800 mol% excess of cyclohexene with a 1 1 molar mixture of sec.-BuLi and f-BuOK in isopentane, followed by addition of oxirane gave the corresponding alcohol in a good yield [1]. Our attempts to metallate cyclohexene with n-BuLi-TMEDA in hexane (20 °C or reflux) or with a 1 1 1 molar mixture of n-BuLi, f-BuOK, and TMEDA in hexane (—20 to + 10°C) gave poor results, presumably due to preferential attack of TMEDA by the base [2]. Reaction (at — 80 to 0 °C) of a five-fold molar excess of cyclohexene with n-BuLi-f-BuOKin a 1 1 mixture of THF and hexane, followed by reaction with oxirane gave the expected alcohol in only 35% yield. A much better result is obtained, however, if first the hexane is removed from the n-BuLi solution by evacuation. Yields in the region of 60% can be obtained, when cyclohexene is used in a four fold excess. 

Reaction of Bis(methylthio)methane with Potassium Amide in Liquid Ammonia and Subsequent Reaction with Oxirane... 

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