D-and L-Allothreonine [D-and L-aThr; (2R,3R)-and (2S,3S)-2-amino-3-hydroxybutanoic acid] are useful as chiral reagents in asymmetric syntheses.1) However, D-and L-aThr, non-proteinogenic a-amino acids, are difficult to produce commercially in large quantities. Therefore, synthetic DLaThr 2-4) has been subjected to optical resolution by separating the diastereoisomeric salts of DL-aThr derivatives to obtain the enantiomers. 5,6) In our previous paper, 7) DL-aThr was found to exist as a conglomerate and to be optically resolved by preferential crystallization and replacing crystallization of DL-aThr. The optical resolution by replacing crystallization is a procedure for obtaining an enantiomer from a conglomerate, and is achieved by allowing an optically active co-solute to coexist in a racemic supersaturated solution.8) DL-aThr was optically resolved using 4-hydroxy-L-proline (L-Hyp) as the optically active cosolute, and D-aThr was allowed to preferentially crystallize from a supersaturated solution of DL-aThr.7) Although L-aThr will be preferentially crystallized from the racemic solution in the presence of D-Hyp, L-Hyp is expensive and D-Hyp is not commercially available. Therefore, we selected D-and L-Ala, which are commercially available and are the most inexpensive optically active a-amino acids, as the optically active cosolutes in optical resolution by replacing crystallization of DL-aThr (Chart 1).Optical resolution by replacing crystallization is based on different interactions between enantiomers and the optically active co-solute. Therefore, the solubilities (mole fractions) of D-, L-, and DL-aThr were first measured in the presence of L-Ala (16.0 mmol) in 100 cm 3 of water at 10°C; these values are summarized in Table 1

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A series of systematic investigations has been conducted to examine the doping effects of dimethyl-tin-dichloride (DMTC) on the structural, electrical, and optical properties of CdS films used for photovoltaic applications. After the DMTC doping, Sn and Cl concentrations in CdS are found to increase to 6×1017 and 4.5×1018atoms∕cm3, respectively; the CdS grain size also tends to increase. A slight increase in the electrical conductivity is discussed in conjunction with the increase in the grain size and the Cl donor concentration. In addition, the optical-absorption spectra reveal almost no change in the band gap of CdS due to the doping, while the spectra indicate a deterioration of the crystallinity of the heavily doped CdS. The influence of the DMTC doping into the CdS films on the formation of the CdS∕CdTe heterostructure is also discussed. With an increase in the grain size of the CdS underlayer due to the DMTC doping, the degree of the (111) preferential orientation of CdTe increases, which generally corresponds to an improvement in the photovoltaic performance. On the other hand, a fairly small reduction is observed in the sulfur fraction in the CdTe1−xSx mixed-crystal interlayer, which indicates no marked deterioration at the CdS∕CdTe interface. The DMTC doping is also effective in reducing the optical reflectance in almost the entire active spectral region of the CdS∕CdTe solar cell.

ethylthio)prepanoic acids [(RS)and (R)-ACE], respectiyely, (RS)-ACE was found to exist as a conglomerate based on its melting point, selubility, and infrared spectrum. (RS)-ACE was optical]y reso]yed by preferential crystallization to yield (R)and (S)-ACE. The obtained (R)and (S)-ACE were eMciently recrystallized frem water, taking acceunt of the solllbility of (RS)-ACE, te give them in optically pure form.

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