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The enzymatic hydrolysis of esters of tertiary alcohols has long been a challenge. In particular their kinetic resolutions have virtually not been addressed. Here we describe the successful kinetic resolution of a,a-disubstituted cyanohydrin acetates, a type of protected tertiary alcohols that form particularly interesting building blocks in organic synthesis. Utilising Subtilisin A the hydrolysis reaction was (S)-selective, while Candida rugosa lipase was (R)-selective. With these commercially available enzymes both enantiomers of the a,a-disubstituted cyanohydrin acetates are now accessible.

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The reactivity of 3-nitro-4-pyridyl isocyanate (7) and 5-nitropyridin-2-yl isocyanate (9) in 1,3-dipolar cycloaddition reactions with azides and pyridine N-oxides has been investigated. 1,3-Dipolar cycloaddition to trimethylsilylazide (TMSA) afforded the respective tetrazolinones, 1-(3-nitropyridin-4-yl)-1H-tetrazol-5(4H)one (8, 50 %) and 1-(5-nitropyridin-2-yl)-1H-tetrazol-5(4H)one (11, 64 %). Respectively, 1,3-dipolar cycloaddition of nitropyridyl isocyanates 7 and 9 to 3,5-dimethylpyridine N-oxide (14), 3-methylpyridine N-oxide (21) and pyridine N-oxide (22) gave the substituted amines, 3,5-dimethyl-N-(3-nitropyridin-4-yl)pyridin-2-amine (17), 3,5-dimethyl-N-(5-nitropyridin-2-yl)pyridin-2-amine (20), N-(5-nitropyridin-2-yl)pyridin-2-amine (24), 5-methyl-N-(5-nitropyridin-2-yl)pyridin-2-amine (23) and 3-methyl-N-(5-nitropyridin-2-yl)pyridin-2-amine (25) in 65 -80 % yield, obtained by cycloaddition, rearrangement and decarboxylation. The results demonstrate that the nitropyridyl isocyanates (7,9) readily undergo 1,3-dipolar cyloaddition reactions similar to phenyl isocyanates.

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The secondary phosphine oxide, t-BuPhHP=O, the most prominent chiral member of this compound class, has been resolved in high yield and with excellent ee. This resolution discloses an efficient route to enantiopure phosphorus compounds.Trivalent chiral phosphorus compounds are of tremendous importance in organic chemistry, especially in asymmetric synthesis 2 and catalysis. Transition metal catalysis uses phosphorus-based ligands extensively 3 whereas phosphorus-based catalysts also play an important role in organocatalysis. 4The synthesis of enantiopure P-chiral compounds, with chirality centered on phosphorus, is in general difficult to achieve. 5 Several successful approaches have nevertheless been developed over the years using chiral auxiliaries such as menthol or ephedrine, or chiral bases such as sparteine. These methods are frequently applied but require several synthetic steps, and the chiral pool sometimes provides only one enantiomer of the auxiliary. The resolution of tertiary phosphine oxides via diastereomeric complex formation (classical resolution) 6 has been reported a number of times as well.Secondary phosphine oxides, 7 (or phosphinous acids, Scheme 1), are important ligands in nonasymmetric transition metal catalysis, especially in cross-coupling reactions. 3,8 This class of ligands combines excellent coordinating properties with low oxidation sensitivity and is easily prepared, often in one step. These properties also make them in principle very good ligands in asymmetric catalysis; 9 secondary phosphine oxides have successfully been used in iridium-catalyzed imine hydrogenation, 10 iridium-and rhodium-catalyzed alkene hydrogenation, 11 palladium-catalyzed allylic substitution, 12 and platinumcatalyzed alkylidenecyclopropanation. 13 Chiral enantiopure secondary phosphine oxides are also excellent starting materials for P-chiral phosphines and other derivatives (Scheme 2). 14 Alkylation of these compounds takes place with retention of stereointegrity, leading to tertiary phosphine oxides. 15 Subsequent deoxygenation affords the corresponding chiral phosphines. 16 The use of the corresponding borane complexes in a variety of applications has recently been described. 17 The barrier in the application of chiral secondary phosphine oxides is the difficulty to obtain them enantiopure. Several strategies have been developed for the preparation of enantioenriched secondary phosphine oxides, mainly Scheme 1 A secondary phosphine oxide (R 1 , R 2 = alkyl or aryl) in equilibrium with its phosphinous acid. For the free compound only the phosphine oxide form is observed, but it tautomerizes to the phosphinous acid form upon complexation with a metal. P R 2 R 1 O H P R 2 R 1 OH Scheme 2 Secondary phosphine oxide 1 as a starting material for chiral enantiopure phosphorus compounds R = alkyl, allyl, aryl CH 2 COOMe, CH 2 OH R = alkyl, aryl (R)-(+)-1 P O H P O R P P O Br P O OR R Downloaded by: University of Illinois at Chicago. Copyrighted material. 2062 J. Holt et al. PAPER Synthesis 2009, No. 12, 2061-206...

Lipase‐catalyzed transesterifications and biological reductions were used to obtain the (S)‐enantiomers of 3‐chloro‐1‐(1,3‐dithian‐2‐yl)‐2‐propanol and 1‐(1,3‐dithian‐2‐yl)‐3‐fluoro‐2‐propanol and their (R)‐acetates. (S)‐3‐Chloro‐1‐(1,3‐dithian‐2‐yl)‐2‐propanol and (R)‐3‐chloro‐1‐(1,3‐dithian‐2‐yl)‐2‐propyl acetate were converted into the respective enantiomers of 2‐(1,3‐dithian‐2‐ylmethyl)oxirane. During this work we also isolated a new gem‐disubstituted epoxide, 2‐(chloromethyl)‐2‐(1,3‐dithian‐2‐yl)oxirane. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)