Microwave-assisted epoxidation of hindered homoallylic alcohols using VO(acac)2: application to polypropionate synthesis
摘要:
The VO(acac)(2) catalyzed epoxidation of hindered homoallylic alcohols was conducted under microwave irradiation in an open vessel using toluene as solvent. The reaction time for the epoxidation of a series of cis- and trans-2-methyl-3-alkenols was dramatically reduced from 6 to 10 days to less than 3 h when compared to conventional heating. The cis alkenols exhibited very high diastereoselectivity. The more elaborated polypropionate precursors 12, 14 and 16 were epoxidized in good yield and excellent diastereoselectivities using the microwave-assisted epoxidation technique described here, which is safe and suitable for multi-gram scales. (C) 2004 Elsevier Ltd. All rights reserved.
Microwave-assisted epoxidation of hindered homoallylic alcohols using VO(acac)2: application to polypropionate synthesis
摘要:
The VO(acac)(2) catalyzed epoxidation of hindered homoallylic alcohols was conducted under microwave irradiation in an open vessel using toluene as solvent. The reaction time for the epoxidation of a series of cis- and trans-2-methyl-3-alkenols was dramatically reduced from 6 to 10 days to less than 3 h when compared to conventional heating. The cis alkenols exhibited very high diastereoselectivity. The more elaborated polypropionate precursors 12, 14 and 16 were epoxidized in good yield and excellent diastereoselectivities using the microwave-assisted epoxidation technique described here, which is safe and suitable for multi-gram scales. (C) 2004 Elsevier Ltd. All rights reserved.
Regioselective cleavage of 3,4-epoxy alcohols with substituted alkynylaluminum reagents: application to the stereoselective synthesis of polypropionates
作者:Wilnelia Dávila、Wildeliz Torres、José A. Prieto
DOI:10.1016/j.tet.2007.05.122
日期:2007.8
reaction of 3-O-substituted propynyl aluminum reagents with a TIPS-protected 2,3-epoxy alcohol and several diastereomeric 2-methyl-3,4-epoxy alcohols offers a convenient synthetic approach for the subsequent preparation and epoxidation of allylic alcohols. The yields are low to moderate and the regioselectivity (internal vs external attack) similar to that of the standard diethylpropynyl aluminum reagent
Stereochemistry of the iodocarbonatation of cis- and trans-3-methyl-4-pentene-1,2-diols: the unusual formation of several anti iodo carbonates
作者:Raymond Tirado、Jose A. Prieto
DOI:10.1021/jo00073a027
日期:1993.10
A study on the stereoselective preparation of a series of 3-methyl-4,5-epoxy alcohols as an entry to polypropionates was undertaken. Initially, the opening of cis and trans TIPS-protected 2,3-epoxy butanols by propynyldiethylalane showed an excellent regioselectivity favoring the monoprotected 1,2-diol products. The resulting propargylic alcohols were stereoselectively reduced to the cis and trans 1-[(triisopropylsilyl)oxy]-3-methyl-4-hexen-2-ols. Iodocarbonatation of these four isomeric homoallylic alcohols was carried out and the stereochemistry of the intermediate iodo carbonates established. Interestingly, a complete anti selectivity (>20:1 anti:syn) was observed when both the syn 3-methyl and cis double-bond geometry were present (3b, 10, and 20). The anti relative configuration for all of the iodo carbonates was established by NMR, and that of 5b was confirmed by X-ray crystallography. This study demonstrated that the relative stereochemistry of the hydroxyl and C(3) methyl groups in combination with the cis or trans geometry of the alkene exerts a significant effect on the stereochemical outcome of the iodocarbonatation reaction. Methanolysis of the iodo carbonates produced the desired 3-methyl-4,5-epoxy alcohols. The application of this chemistry to the reiterative synthesis of polypropionates was carried out with epoxy alcohol 4a (anti isomer), producing a new homologated epoxy alcohol, 22, with six contiguous stereocenters in a highly stereoselective manner.
Efficient Hydroxyl Inversion in Propionates via Cesium Carboxylates
作者:David O. Arbelo、Leslie Castro-Rosario、José A. Prieto
DOI:10.1081/scc-120023443
日期:2003.9
A versatile method for hydroxyl inversion, based on cesium carboxylates, was developed for polypropionate precursors and models. This procedure is compatible with a number of functional groups, including 3,4-epoxy alcohols, which tend to cyclize under basic conditions. This strategy allowed the preparation of a series of diastereomeric alcohols and new 3,4-epoxy alcohols, not available by standard iodocyclization techniques.
Regioselective cleavage of cis- and trans-2-methyl-3,4-epoxy alcohols with diethylpropynyl aluminum
作者:Raymond Tirado、Gerardo Torres、Wildeliz Torres、José A. Prieto
DOI:10.1016/j.tetlet.2004.12.018
日期:2005.1
The regioselectivity of the reaction of diethylpropynyl aluminum with diastereomeric 2-methyl-3-4-epoxy alcohols was studied. The preferred side of attack (1,3-diol vs 1,4-diol product) depends on the stereochemical disposition of the substituents. NMR studies showed that the regiochemistry of this reaction is governed by the aluminum coordination pattern. Protection of the alcohol with MEM provides the 1,3-diol product in systems where the free alcohol produced the 1,4-diols. (C) 2004 Elsevier Ltd. All rights reserved.
Microwave-assisted epoxidation of hindered homoallylic alcohols using VO(acac)2: application to polypropionate synthesis
作者:Gerardo Torres、Wildeliz Torres、José A. Prieto
DOI:10.1016/j.tet.2004.08.095
日期:2004.11
The VO(acac)(2) catalyzed epoxidation of hindered homoallylic alcohols was conducted under microwave irradiation in an open vessel using toluene as solvent. The reaction time for the epoxidation of a series of cis- and trans-2-methyl-3-alkenols was dramatically reduced from 6 to 10 days to less than 3 h when compared to conventional heating. The cis alkenols exhibited very high diastereoselectivity. The more elaborated polypropionate precursors 12, 14 and 16 were epoxidized in good yield and excellent diastereoselectivities using the microwave-assisted epoxidation technique described here, which is safe and suitable for multi-gram scales. (C) 2004 Elsevier Ltd. All rights reserved.