endo-Fused lactone 3 was obtained in high yield from the mechanism of its formation and reactivity were investigated. Lactone 3 serves as key intermediate for lactol 1 which is a camphoracetic acid 2 with thionyl chloride and a subsequent reduction of intermediate 5 with tributyltin hydride. The useful reagent in racemate resolution and asymmetric synthesis. structure of 5 was elaborated and some aspects of the The enantiomerically pure lactol 1 [1] is a very versatile higher than was to be expected from the original ratio 5/6, we assumed that during the crystallization a further equili-anomer-selective protecting group and chiral auxiliary. The advantageous application of 1 in racemate resolution [2] [3] [4] bration had occurred. As a consequence, we decided to concentrate the mother liquor, the residue was dissolved in and asymmetric synthesis [2] [5] has been demonstrated many times, and acetals derived from 1 have been used for studies CDCl 3 , and the time dependence of the ratio 5/6 was measured (Table ). of stereoelectronic effects . In addition, 1 allows the determination of the absolute configuration [2] [3] [7] and the enan-Although no crystallization occurred during the experitiomeric purity [2] [3] of alcohols, cyanohydrins, thiols, acids, ment, the equilibrium between 5 and 6 was far in advantage and amines.
of the chlorolactone. Thus, to the product of this reaction Despite this broad range of useful applications, a new should be assigned structure 5 in contrast to earlier assumpsynthesis was described only recently which allows to manutions [9] . facture 1 in a high overall yield from d-camphor on a multi-First hints for the cyclic structure of 5 were obtained kilogram scale (see Scheme 1) . A key step in this reaction from IR [10] and 13 C-NMR data which allowed to dissequence is the sodium tetrahydroborate reduction of the tinguish between open-chain keto acid chlorides and cyclic endo-camphoracetic acid 2 yielding Ϫ rather unselectively pseudoacid chlorides. In the IR spectra only one carbonyl Ϫ endo-lactone 3 and the trans-isoborneolacetic acid 4, signal was observed whereas 2 showed two absorption which for economical reasons, has to be recycled. This can bands in this region. In the 13 C-NMR spectra there was no be done very easily by reoxidation, but, especially on a labrecognizable signal for the camphor carbonyl group, but a oratory scale, this requires at least two more steps. We have new signal at a chemical shift, which one would expect for thus developed a new two-step sequence for the synthesis an O,Cl acetal carbon atom, appeared. In our case no defiof 3 from 2 which avoids the formation of 4.
nite assignment was possible from 1 H-NMR data . Eventually, the structure of 5 was proven by X-ray analysis (Figure ).
Results and Discussion
The formation of chlorolactones from γ-keto acids has received considerable attention in the past Ϫ especially in Our new method takes advantage of chlorolactone 5. This pseudoacid chloride is obtained as the main product the case of laevulinic acid chloride [13] and some 1,2-dicarboxylic acid chlorides [14] Ϫ but is now well precedented [15] . when 2 is treated with thionyl chloride and only about 12% of open-chain isomer 6 is observed. Treating this mixture However, concerning the mechanism of the chlorolactone formation, it was proposed only at one instance that the with n-hexane affords colorless crystals of 5 in 94% yield based on 2. Since the isolated yield of crystalline 5 was carbonyl oxygen atom simply displaces the halogen