(Z,1S,10aR)-(−)-Menthyl 1-hydroxy-1,2,3,5,6,7,10,10a-octahydropyrrolo[1,2-a]azocine-10a-carboxylate

The structure determination confirms the stereochemistry of the title compound, C21H35NO3, obtained as an intermediate in the enantioselective synthesis of deoxynojirimicine analogs. The system contains a pyrrolo[1,2-a]azocine backbone, which was synthesized by a domino process involving a [2,3]-sigmatropic rearrangement. The incorporation of a chiral auxiliary (−)-menthyl, whose stereocentres are not involved during the synthesis, enables the assignation of absolute configuration. The crystal structure features O—H⋯O hydrogen bonds involving the hydroxy groups as donors and the carbonyl groups as acceptors, which link the molecules into chains running along [010].

The structure determination confirms the stereochemistry of the title compound, C 21 H 35 NO 3 , obtained as an intermediate in the enantioselective synthesis of deoxynojirimicine analogs. The system contains a pyrrolo[1,2-a]azocine backbone, which was synthesized by a domino process involving a [2,3]sigmatropic rearrangement. The incorporation of a chiral auxiliary (À)-menthyl, whose stereocentres are not involved during the synthesis, enables the assignation of absolute configuration. The crystal structure features O-HÁ Á ÁO hydrogen bonds involving the hydroxy groups as donors and the carbonyl groups as acceptors, which link the molecules into chains running along [010].

Experimental
The used route for the synthesis of the title compound is described in Figure 1. The diazocarbonyl derivative 1 was synthesized starting from L-proline and (-)-menthyl acetate. The decomposition in refluxing toluene with Cu(acac) 2 or Rh 2 (OAc) 4 brought in one step to the pyrroloazocine alkaloid 3. The decomposition triggers a domino process that involved a carbenoidic attack to the nitrogen lone pair and the formation of the [5,5]-spirocyclic ammonium ylide 2. The ylide undergoes a [2,3]-sigmatropic rearrangement and it was possible to isolate the alkaloid 3 in 70% yield and 97% enantiomeric excess (Muroni et al., 2006). The stereo specific nature of [2,3]-sigmatropic rearrangement allows a complete transfer of chirality (Sweeney, 2009;Zhang & Wang, 2010). As first step in the conversion in deoxynojirimicine analogs (Asano et al., 2000;Watson et al., 2001), the reduction of the carbonyl with L-selectride gave, after chromatography and recrystallization, the title compound 4 as a single diastereoisomer. The structure determination confirms the configuration of the quaternary stereocentre formed during the domino sequence and the configuration of the carbinol function, which is in accordance with the attack of L-selectride at the opposite face of the ester function.
The structural model ( Fig. 2) showed standard bond lengths and angles; the X-ray analysis confirmed a cis C7═C8 double bond in the azocine ring, and the stereochemistry of C atoms known from literature in the auxiliary chiral (-)menthyl: S-C13, R-C12 and R-C16. Two new chiral centers were identified S-C1 and R-C10.
The crystal structure ( Fig. 3 and 4) consists of one type of O-H···O hydrogen-bond, with each molecule acting as a donor and acceptor of two hydrogen bonds. One molecule is linked through hydrogen interaction to other two symmetryrelated molecules in the crystal, resulting in the formation of chains parallel to the [010] direction.

Experimental
All 1 H NMR (400 MHz) and 13 C NMR (100 MHz) spectra were recorded on a Varian Mercury plus 400 spectrometer.