Regioselective opening

Oxiranes with a single perfluoroalkyl chain are regioselectively opened by nucleophilic reagents at the more accessible carbon [d4] (equation 30). 

Our journey begins with the photo-induced union of 3,4-dimethylfuran (19) and / -(benzyloxy)-propanal (18) (see Scheme 5). Irradiation of a solution of these two simple, achiral compounds in benzene with a 450 W Hanovia lamp equipped with a vycor filter results in the exclusive formation of head-to-head, exo photoadduct 17 in 63% yield. As a cw-fused dioxabicyclo[3.2.0]heptene system, intermediate 17 possesses a folded molecular framework to which access is obstructed on the concave face. In the presence of mCPBA, the less hindered convex face of the enol ether double bond is oxidized in a completely diastereoselective fashion, affording intermediate 16 in 80% yield after regioselective opening of... 

Similarly, the synthesis of (+)-T-7 (129) was achieved in a five-step sequence from [2R-(2R)]-22l in 32% overall yield (Scheme 8). The regioselective opening of the epoxide of [2R-(2R)]-223 with lithium dibutylcuprate afforded alcohol [2R-(2R)]-227 that was transformed in a three-step sequence into (+)-T-7 (129) in 51% yield. In an analogous way (+)-T-8 (126) was obtained from [2R-(2S)] 223 in four steps and 35% yield. 

Direct epoxidation of compound 20 with Bu OOH in the presence of VO(acac)2 [10], proceeded chemoselectively to give the epoxide 21 which was regioselectively opened with UAIH4 to afford the 1,2-diol 22 (76%). X-ray diffraction analysis of this compound confirmed the assigned stereochemistry for intermediates 20-22. 

A three-step protocol has been applied to obtain homoallylic carbanions from cyclopropylmethanol derivatives by regioselective opening of intermediate cyclopropyllithiums. ... 

The preparation of the requisite y-keto-p-toluenesulfonate rac-35 as homo-Favorskii precursor commenced with commercially available 2,5-dihy-droanisole (36) that was protected and epoxidized to acetal rac-31 (Scheme 11). Regioselective opening of the epoxide with p-chlorophenylse-lenide followed by sequential oxidation to the selenoxide and thermal elimination generated an allylic alcohol that was protected to give pivaloate rac-38. 

Highly regioselective opening of epoxides and aziridines with sodium azide and Oxone in aqueous acetonitrile in very high yield was reported (equation 52). ... 

An improved method for the synthesis of a-fucosyl (l- 3)-3-thio-glycoside (65 c) involved the nucleophilic displacement of a cyclic sulfamidate derived from allosamine (67). The regioselective opening of (67) with (30b) led to (65c) in good yield (Scheme 21) [51]. 

The question then arises if a regioselective opening of a 2,3-rraMS-epoxy carboxamide derived from aldonic acids would occur. The 2,5-di-O-tosyl-D-ribono-1,4-lactone (62) (Scheme 13) was used to find an answer to this question. If treated with ammonia, the 2,5-diamino-2,5-dideoxy-D-ribono-1,5-lactam (63) was obtained as the only product [79]. The NMR spectra of the reaction mixture showed the formation of the diepoxy amide A which was opened at C-5 by ammonia. In this case no internal lactamization could occur, due to the trans 2,3-epoxy group in B (Scheme 13). Thus, a regioselective opening of an acyclic 2,3-epoxy carboxamide took place at C-2. The reaction was complete within 6 days. 

Regioselective opening of the epoxides 29 was realized with various nucleophiles (CN, N3", OH"). Instead of a two-step process, direct conversion of chlo-rohydrin (S) -28a yielding cyanohydrin 30 was accomplished in 51% yield (Scheme 2.2.7.16) [45]. 

Advantageous use of homochiral cyclohexadiene-cis-l,2-diol, available by means of biocatalytic oxidation of chlorobenzene with toluene dioxygenase, has enabled the synthesis of all four enantiomerically pure C18-sphingosines (Nugent, 1998), which are known inhibitors of protein kinase C and important in cellular response mediation for tumor promoters and growth factors. The four requisite diastere-omers of azido alcohol precursors were accessed by regioselective opening of epoxides with either azide or halide ions. 

The various explanations that were provided to rationalize the observed re-gioselectivity involved (a) electronic repulsions between the lone pairs of the heteroatoms and the negatively charged oxygen of the perepoxide, (b) anchimeric assistance by the allylic substituent leading to regioselective opening of the possible perepoxide intermediate [Eq. (13)],... 

Chakraborty, T. K. Das, S. Synthesis of chiral 1,3-diols by radical-mediated regioselective opening of 2,3-epoxy alcohols using Cp2TiCl2. Tetrahedron Lett. 2002, 43, 2313-2315. Nakajima, N. Tanaka, T. Hamada, T. Oikawa, Y. Yonemitsu, O. Highly stereoselective total synthesis of pikronolide, the aglycon of the... 

C—C Bond formation with allylmagnesium chloride. J. Org. Chem. 1997, 62, 9342—9344. Overman, L. E. Renhowe, P. A. Regioselective opening of terminal epoxides with 3-(trialkyl-silvljallvl organometallic reagents. J. Org. Chem. 1994, 59, 4138-4142. 

Charette, A. B. Naud, J. Regioselective opening of substituted (cyclopropylmethyl)lithiums derived from cydopropylmethyl iodides. Tetrahedron Lett. 1998, 39, 7259-7262. 

To circumvent elimination reactions, fluorides have been introduced by nucleophilic ring opening of epoxides.15 The regioselective opening of aldosyl epoxides proceeds ira r-diaxially.16 Epoxides can also be opened with chloride, bromide and iodide ions. The chemistry of epoxides is discussed in detail in Section 3.6. 

Vinyl epoxides can be regioselectively opened depending on the substitution at the 2-position of the 2-lithio-l,3-dithiane. Thus, the Sw2 adducts anti-204 were obtained with unencumbered dithiane anions, whereas Sa 2 adducts 205 were produced with sterically encumbered ones (Scheme 58)293. 

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