2 , 5-Anhydro-N-benzyl-2-C-methyl-D-arabinonamide [ ( 2 S , 3 R , 4 R )-N-benzyl-3 , 4-dihydroxy-2-methyltetra-hydrofuran-2-carboxamide ]

# 2007 International Union of Crystallography All rights reserved The size of the ring and relative configuration of the chiral centres in the title compound, C13H17NO4, formed by the preferential formation of the hindered five-membered ring tetrahydrofuran rather than the expected three-membered ring epoxide, was established by X-ray crystallographic analysis; the absolute configuration was determined by the use of 2-C-methyl-d-arabinono-lactone as the starting material. The crystal structure consists of hydrogen-bonded layers lying with their hydrophobic surfaces in contact.

The size of the ring and relative configuration of the chiral centres in the title compound, C 13 H 17 NO 4 , formed by the preferential formation of the hindered five-membered ring tetrahydrofuran rather than the expected three-membered ring epoxide, was established by X-ray crystallographic analysis; the absolute configuration was determined by the use of 2-C-methyl-d-arabinono-lactone as the starting material. The crystal structure consists of hydrogen-bonded layers lying with their hydrophobic surfaces in contact.
It was thus expected that treatment of the tosylate (3) of 2-C-methyl-d-arabinonolactone (Hotchkiss et al., 2006) would give the l-xylono epimer (6); however, a complex mixture of products was obtained. Accordingly the reaction sequence treatment of (3) with benzylamine was expected to give ring opening of the lactone unit to (4) which would be followed by formation of the epoxide (5) from attack of the C4 hydroxyl group; (5) could be subsequently closed to the target (6). A product was isolated from the reaction of benzylamine with (3) in 61% yield. X-ray crystallographic analysis showed that the much hindered tertiary alcohol at C2 of (4) had closed to form the tetrahydrofuran (7). The connectivity of the C and H atoms is the same in both (5) and (7), and the X-ray experiment unequivocally established that the five-membered ring THF (7) was formed in preference to the three-membered ring epoxide (5); the absolute configuration of (7) is determined by the use of 2-C-methyl-d-arabinonolactone as the starting material.
The molecular structure ( Fig. 1) shows no abnormal features, even a short internal N-HÁ Á ÁO contact (Table 1) having no visible influence [largest distance deviation from the MOGUL norms (Bruno et al., 2004) is C1-O5 (1.46 vs 1.43 Å ), largest angle deviation is C11-C16-C10 (122.8 vs 120.8 )]. The crystal structure consists of hydrogen-bonded sheets (Fig. 2). Both faces of the sheets are composed largely of phenyl groups which lie in contact in the crystal structure (Fig. 3).

Experimental
The synthesis of (3) is described in the Comment and shown in the scheme; full details will be reported elsewhere. The sample for analysis was crystallized from a 2:1 mixture of ethanol and methanol to yield colourless needles with m.p. 402-404 K and [] D 19 = À18.5 (c =1.00, CH 3 OH).  Partial packing diagram of the title compound showing a single hydrogenbonded (dotted lines) layer lying perpendicular to the c axis. Each molecule is involved in only two hydrogen bonds.

Figure 3
Partial packing diagram viewed perpendicular to the plane of the molecular sheets showing hydrophobic (largely aromatic) plane-to-plane contacts at (x, y, 1 2 ). Hydrogen bonds are shown as dotted lines.
In the absence of significant anomalous scattering, Friedel pairs were merged and the absolute configuration assigned on the basis of the starting material.
The H atoms were all located in a difference map, but those attached to C atoms were repositioned geometrically. The H atoms were initially refined with soft restraints on the bond lengths and angles to regularize their geometry (C-H in the range 0.93-0.98, N-H to 0.86, O-H = 0.82 Å ) and U iso (H) (in the range 1.2-1.5 times U eq of the parent atom), after which the positions were refined with riding constraints.