After recrystallization from chloroform, the crystals of Ib contain one molecule of CHC13 per cage molecule, which they slowly lose at room temperature. The structure analysis was therefore carried out at 140 K. 1161 S . S P Parkin, E. M. Engler, R. R. Schumaker, R. Lagier, V. A. Lee, J. 1610 R'OH =-1 6-11 c, n = 3; d, n = 5 1380 0 VCH Verlagsgesellschafr mbH. 0-6940 Weinheim. 1988

The design of substituted lactols is described, which in their reaction with racemic alkyl aryl carbinols react preferentially with one of the enantiomers exhibiting selectivities up to 14:1.

Protected 2'-O-methyl and 2'-O-ethyl ribonucleoside-3'-O-phosphoramidites were prepared via alkylation of the ribonucleosides at an early stage in the synthesis. Utilizing a strategy of minimal protection, the alkylation was performed with unprotected cytidine and adenosine, or with O6-protected guanosine and N3,5'-O-protected uridine using methyl or ethyl iodide and sodium hydride. In subsequent steps, the introduction of standard protective groups for oligonucleotide synthesis and the concomitant separation from 3'-O-alkylated isomers was accomplished. A modification of the phosphitylation procedure permitted facile isolation of the desired phosphoramidites which show high coupling efficiencies in oligomer assembly.

Condensation of d-camphor (1) with diethyl oxalate and subsequent ester hydrolysis yielded camphoroxalic acid 3. endoFused lactone 9 was obtained by reduction of 3 with zinc followed by reduction with NaBH, and ring closure with H2S04. exo-Fused lactone 16, the crystal structure of which was determined, was prepared by reduction of 3 with NaBH4 under basic conditions followed by catalytic hydrogenation and ring closure with HCl. Reduction of 9 and 16 with DI-BALH and application of acidic work-up conditions yielded the self-condensed reduction products (lactol anhydrides) 10 and 17, which are useful reagents in racemate resolution and stereoselective synthesis.In the reaction of enantiomerically pure lactols A['] and Bt2] with alcohols acetal centers with an absolute configuration corresponding to that of the glucosidic centers of a-D-glucose (A type) and a-L-glucose (equal to P-D-glucose; B type), respectively, are formed stereoselectively. This selectivity is achieved by endo (A) and exo fusion (B), respectively, of the lactol ring with the bornane ring system, which results in blockage of one of the sides of the lactol ring (see Scheme l)13]. These compounds were used for studies of stereoelectronic effectsL41, for racemate res~lution[~-~] and asymmetric and allow the determination of the absolute c o n f i g u r a t i~n [~~~~~] and enantiomeric purity [s,6] of alcohols, cyanohydrins, thiols, acids, and amines.An eficient synthesis of these compounds is a prerequisite for their use in synthetic chemistry, including their most general application as protective groups of the acetal type (such as dihydro-2H-pyran). The procedure ['] for the synthesis of compound A is acceptable, although it includes five steps. The synthesis[',*] of compound B, however, which is in general a reagent superior to A in makes use of a by-product of the synthesis of A and requires at least one purification step by column chromatography. Therefore, its range of application is extremely limited. For this reason new methods for the synthesis of the anhydrides 10 and 17 of both compounds were developed, Lo] Part XIII: Ref. [9b] which have been shown to be eficient also on a multikilogram scale of manufacture. Results and DiscussionBecause camphor condensation reactions in a position to the carbonyl group give higher yields than alkylations[lOl, oxalic ester was chosen as reagent to introduce all carbon atoms required for ring formation in one step. Ethyl camphoroxalate 2 was prepared by the known procedure [' '] for the condensation of sodium camphorate with diethyl oxalate. Xylene was used as solvent instead of diethyl ether[l21, thus allowing a faster and far more efficient deprotonation of camphor at elevated temperatures. The formation of compound 4 as by-product, which was confirmed by an experiment at 90 "C, was suppressed by rigid control of temperature and by application of an excess of diethyl oxalate.Treatment of ethyl ester 2 with potassium hydroxide yielded camphoroxalic acid 3 in very good yield. Both 'Hand 13C-NMR sp...

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