Oxirane reactions reduction

Synthesis of All-t/a/is-Geranylgeraniol. The type of alkylation described above for the synthesis of bacillariolide III is widely used in the synthesis of natural products due to the mild reaction conditions and high stereospecificity. The formation of the C-C bond takes place when activated a-sulfonyl car-banions derived from (3-ketosul tones, a-sulfonyl sulfones or, less often, allylic sulfones react with the H-allyl palladium complex. In the synthesis of all-trans-geranylgeraniol, the a-sulfonyl carbanion adds to the Ti-allylpalladium complex of 2-(prop-l-en-2-yl)oxirane. Final reductive desulfonylation affords the desired compound, as depicted in Eq. 147.254... 

Several bi- and tri-cyclic thiepan derivatives were obtained by autoxidation of the cyclobutadiene (111) or by various reactions with strained sulphur-containing acetylenes, e.g.. the dithiet (112). The ylides derived from cis- and trans-ill) were alkylated stereoselectively at low temperatures, and they reacted with carbonyl compounds to give ring-opened oxirans (113). Reduction of the 6-methoxy-carbonyl derivative of (72) afforded the ten-membered cyclic sulphide (114). ... 

Suami s group has synthesized the isomeric 3,4-, 2,5-, 4,5-, and 4,6-dideoxy-streptamines using standard reactions of either azide ion or hydrazine on dideoxy-inositol sulphonates or oxirans. The reduction of l,2 3,4-dianhydroinositoIs and 1,2-anhydroinositol sulphonates with lithium aluminium hydride has furnished a series of dideoxy- or 1,4-anhydrodeoxy-inositols e.g., see Scheme 117), 1-Amino-1-deoxy-wyo-inositol has been synthesized by way of an azide displacement on 1l-1, 2 3,4-di-0-cyclohexylidene-5-0-methyl-6-0-toluene-/7-sul-phonyl-cA/ra-inositol. ... 

Ring opening of the sulfur-free oxiranes 76, prepared from 68 by Pummerer reaction, reduction of the intermediate aldehyde and subsequent 0-benzylation, by action of nucleobases, for example thymine and fluorouracil, provided an efficient entry to fluoronucleosides 77 (77) (Scheme 18). 

Terminal alkynes react with propargylic carbonates at room temperature to afford the alka-l, 2-dien-4-yne 14 (allenylalkyne) in good yield with catalysis by Pd(0) and Cul[5], The reaction can be explained by the transmetallation of the (7-allenylpailadium methoxide 4 with copper acetylides to form the allenyKalk-ynyl)palladium 13, which undergoes reductive elimination to form the allenyl alkyne 14. In addition to propargylic carbonates, propargylic chlorides and acetates (in the presence of ZnCb) also react with terminal alkynes to afford allenylalkynes[6], Allenylalkynes are prepared by the reaction of the alkynyl-oxiranes 15 with zinc acetylides[7]. 

Reductive cleavage of oxiranes to alcohols by lithium aluminum hydride is an important reaction (64HC(19-1)199), but the most powerful hydride donor for this purpose is lithium triethylborohydride (73JA8486). 

Dihydro-2f/-pyran-2-one has been prepared by reductive cycliza-tion of 5-hydroxy-2-pentynoic acid [2-Pentynoic acid, 5-hydroxy-], which is obtained in two steps from acetylene [Ethyne] and ethylene oxide [Oxirane] 3 and by the reaction of dihydropyran [277-Pyran, 3,4-dihydro-] with singlet oxygen [Oxygen, singlet].4,5 2ff-Pyran-2-one has been prepared by pyrolysis of heavy metal salts of coumalic acid [2//-Pyran-5-carboxylic acid, 2-oxo-],8 by pyrolysis of a-pyrone-6-carboxylic acid [211 - Pyran-6-carboxyl ic acid, 2-oxo-] over copper,7 and by pyrolysis of coumalic acid over copper (66-70% yield).8... 

Oxidation 205-213 /J-Oximinosulphoxides, chiral 336 Oxiranes 169 reactions of 305 synthesis of 639 2-Oxo-l,2,3-oxathiazolidines 71 /1-Oxosulphones, synthesis of 169 Oxosulphonium ylides, reactions of 219 /3-Oxosulphoxides reduction of 347-349 synthesis of 337-340... 

The stereochemistry of the first step was ascertained by an X-ray analysis [8] of an isolated oxazaphospholidine 3 (R = Ph). The overall sequence from oxi-rane to aziridine takes place with an excellent retention of chiral integrity. As the stereochemistry of the oxirane esters is determined by the chiral inductor during the Sharpless epoxidation, both enantiomers of aziridine esters can be readily obtained by choosing the desired antipodal tartrate inductor during the epoxidation reaction. It is relevant to note that the required starting allylic alcohols are conveniently prepared by chain elongation of propargyl alcohol as a C3 synthon followed by an appropriate reduction of the triple bond, e. g., with lithium aluminum hydride [6b]. 

The beneficial effect of added phosphine on the chemo- and stereoselectivity of the Sn2 substitution of propargyl oxiranes is demonstrated in the reaction of substrate 27 with lithium dimethylcyanocuprate in diethyl ether (Scheme 2.9). In the absence of the phosphine ligand, reduction of the substrate prevailed and attempts to shift the product ratio in favor of 29 by addition of methyl iodide (which should alkylate the presumable intermediate 24 [8k]) had almost no effect. In contrast, the desired substitution product 29 was formed with good chemo- and anti-stereoselectivity when tri-n-butylphosphine was present in the reaction mixture [25, 31]. Interestingly, this effect is strongly solvent dependent, since a complex product mixture was formed when THF was used instead of diethyl ether. With sulfur-containing copper sources such as copper bromide-dimethyl sulfide complex or copper 2-thiophenecarboxylate, however, addition of the phosphine caused the opposite effect, i.e. exclusive formation of the reduced allene 28. Hence the course and outcome of the SN2 substitution show a rather complex dependence on the reaction partners and conditions, which needs to be further elucidated. 

Basic solid liquid two-phase conditions with f-butyl peroxide and N-benzylquininium chloride convert cyclohex-2-enone preferentially into the 2(S),3(S)-oxirane (20% ee) which, upon purification and treatment with hydrazine, yields (S)-cyclohex-2-enol [7]. This reaction contrasts with the direct reduction of cyclohex-2-enone to the /J-isomer by lithium aluminium hydride in the presence of quinine [20]. 

Finally, a reaction that clearly shows the electrophihc carbenoid-type character of a-lithiated epoxides is the reductive alkylation discovered by CrandaU and Apparu. The transformation is illustrated by the treatment of f-butyl ethylene oxide with t-butyllithium to yield ii-di-f-butylethene (equation 55). The overall reaction results in a conversion of an oxirane into an aUcene under simultaneous substitution of an a-hydrogen atom by the alkyllithium reagent ... 

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