Vinyloxirane

Isopropenyl)oxirane (10.113, Fig. 10.26) and 2-methyl-2-vinyloxirane (10.114) were hydrolyzed by EH to the corresponding diols (10.116 and 10.117, respectively). Nucleophilic ring opening took place at the less-hindered, unsubstituted C-atom, with retention of configuration at C(2). (2R)-2-(Isopropenyl)oxirane was a better substrate than the (25)-enantiomer. Substrate enantioselectivity was more modest in the hydration of 2-methyl-2-vin-yloxirane (10.114), since this compound is chemically more reactive and undergoes partly nonenzymatic hydrolysis. 

Catalytic cycloisomerization of cis-l-ethynyl-2-vinyloxiranes vas implemented by [Ru(Tp)(PPh3)(CH3CN)2]PF6, which afforded substituted phenols, as shown in... 

Scheme 6.20. This ruthenium catalyst (10 mol%) was active for the cydization of ds-1 -ethynyl-2-vinyloxiranes to afford various 2,6-disubstituted phenols in reasonable yields. Under similar conditions, 1,1,2,2,-tetrasubstituted oxiranes gave the 2,3,6-trisubstituted phenols with a skeleton reorganization [22]. The 1,2-deuterium shift of the alkynyl deuterium of d-Sle was indicative of mthenium vinylidene intermediates (Scheme 6.20).

Asymmetric cycloaddition of 2-vinyloxiranes to carbodiimides proceeds in the presence of Pd2(dba)3-CHC13 and TolBINAP as the chiral ligand in THF at room temperature to yield 4-vinyl-l,3-oxazolidin-2-imines with up to 95% ee [74] (Eq. 8A.50). The enantio-determining step is assumed to be the nucleophilic attack of a nitrogen nucleophile on a 7i-allylpalladium intermediate. Reactions of 2-vinyloxiranes with isocyanates using the same catalyst system afford 4-vinyl-l,3-oxazolidin-2-ones with low enantioselectivity. 

The major products of the Sn2 reaction between the sulfur of thiocamphor (18) and (R )-2-vinyloxirane (19) form regioselectively at the most hindered carbon when the reaction is carried out in the presence of the Lewis acids ZnCH, SiC>2, or SnCl4.30 When the reaction is carried out under basic conditions with NaH in anhydrous THF, however, the major product is formed when the sulfur of the enethiolate ion attacks the least substituted carbon in an SN2 reaction (Scheme 9). The initial products from both reactions react further giving more complex products. 

The 2-vinyloxiranes undergo Pd-catalysed furan annelation with soft nucleophiles [36,37], The allenylpalladium 170 is generated from ethynyloxirane 169, and the attack of acetoacetate anion 153 at the central carbon of 170 forms 171. The oxygen... 

Lautens, M. Ouellet, S. G. Raeppel, S. Amphoteric character of 2-vinyloxiranes synthetic equivalents of /3,y-unsaturated aldehydes and a vinylogous enolate. Angew. Chem. Int. Ed. 2000, 39, 4079 1082. 

Thioketones such as 4,4/-dimethoxythiobenzophenone react with enantiopure (R)-2-vinyloxirane to give virtually enantiopure (.S )-4-vinyl-1,3-oxalhiolanes (86) in high yield, using mild Lewis acid conditions (SiC>2, DCM, 0 °C).337 More hindered thioketones require a stronger Lewis acid (BF3, —78 °C). The clear inversion at the oxirane C(2) indicates an efficient. S n2 mechanism. 

Unsaturated aldehydes are poor substrates for allylation because of their inherent instability toward alkene isomerization accordingly, very few reports describe the allylation of these substrates. However, Lautens et al. were able to overcome this problem via an indirect approach by the use of 2-vinyloxiranes 219 as surrogates for the above-mentioned aldehydes. The in situ formation of these otherwise unstable aldehydes is achieved via the Lewis acid treatment of vinyloxiranes to furnish aldehydes 221 which further undergo allylation with chiral allyl boronates 174 in moderate ee (Scheme 37) <20020L83>. The chiral induction could be further enhanced by the use of allylboranes derived from a-pinene. 

Vinyloxiranes have been found to be excellent surrogates to /3,7-unsaturated aldehydes in the preparation of bis(homoallylic) alcohols. Sc(OTf)3 catalyzed the reaction at -78 °G (Equation (165)).640... 

Aside from reactions involving heterocumulenes, processes involving simple activated olefins as dipolarophile partners have been only scarcely investigated. The popular 2-vinyloxiranes [27] as well as N-tosyl-2-vinylaziridinc 20 [28] have been shown to react with Michael acceptors having neces-... 

The addition of 2-methyl-2-vinyloxirane to / -keto esters in the ptesence of a palladium catalyst and a chiral phosphine ligand proceeds regio- and stereoselectively to give 2-hydroxytetrahydrofuran-3-carboxylates (Equation 93) <2001JA12907>. 

Vinyloxirans react with carbodiimides in the presence of Pd(dba)3 to give iminooxazolidines in good yields. Better yields are obtained when unsymmetrical carbodiimides are used in this reaction. 

In the thermally-induced rearrangement of 2-vinyloxirane (Eq. 382), dihydro-furan is formed by thermolysis of the oxirane C-C bond via an ylide intermediate. 

Vapor phase thermolysis of 2,5-hexadien-l-ols at 260°C affords cyclopropylmethyl ketones . The reaction has been proposed to proceed via homodienyl migration of hydrogen and is stereospecific (equation 118). l-(l-Alkynyl)-2-vinyloxiranes undergo successive Cope rearrangements at 300-350°C to give 2-(l-alkynyl)cyclopropane-carbaldehydes (equation 119) . Acid-catalyzed intramolecular acetalization followed by... 

Vinyloxiranes 206 undergo palladium-catalyzed furan annulation with soft nucleophiles via two cyclization paths to give either 210 or 211 as a primary product, which undergoes further transformations, depending on the substituents (Scheme 11-55) [44,47]. At first, the allenylpalladium 207 is generated this is converted to two kinds of n-... 

A sample of ethyl (2/ , 32 )-3-phenyl-2-vinyloxirane-2-carboxylate (100 mg, 5 mmol) was evaporated at 50 °C/0.02 Torr through a horizontally situated Vycor tube (41 cm,5 mm i.d.) heated to 500 °C. The pyrolysate was condensed in a cold trap with cooled liquid Nj. The apparatus was washed with CHjClj and the solvent evaporated to give the product yield 95 mg (95%) Rj 0.42 (10% EtOAc/hexane). 

Z)-3-Chloroallyldiisopinocampheylborane is a valuable allylating agent for aldehydes. The products are syn-chlorohydrins, which can be transformed into. it-l-alkyl-2-vinyloxiranes. a-Allenic alcohols are prepared from propargylic di i sopinocamphey Iboranes. ... 

Although this reaction is still at an early stage of development, a few modifications have been exploited. These modifications include the extension of the Bellus-Claisen rearrangement to ring strain molecules (such as 2-vinyloxetane, 2-vinyloxirane, and 2-vinylaziridines ) and to tertiary allyl amines, the application of chiral Lewis acid derived from Mgl2 and bis(oxazolinyl)aryl ligands, and the implementation of electron-rich ketene equivalents (e.g., alkoxyketenes, aminoketenes) by the photolysis of chromium carbene complexes. 

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