tert-Butyl N-[1-diazoacetyl-3-(methylsulfanyl)propyl]carbamate

In the enantiomerically pure title compound, C11H19N3O3S, the chain C—N—C(O)—O—C—C (from the asymmetric carbon to a methyl of the tert-butyl group) displays an extended conformation. In the crystal, molecules are linked into chains parallel to the c axis by classical N—H⋯Odiazocarbonyl hydrogen bonding and an unusual intermolecular three-centre interaction involving the amino acid (aa) carbonyl Oaa and the diazocarbonyl grouping C(O)—CH—N N, with H⋯Oaa = 2.51 Å and N⋯Oaa = 2.8141 (14) Å.

In the enantiomerically pure title compound, C 11 H 19 N 3 O 3 S, the chain C-N-C(O)-O-C-C (from the asymmetric carbon to a methyl of the tert-butyl group) displays an extended conformation. In the crystal, molecules are linked into chains parallel to the c axis by classical N-HÁ Á Á O diazocarbonyl hydrogen bonding and an unusual intermolecular three-centre interaction involving the amino acid (aa) carbonyl O aa and the diazocarbonyl grouping C(O)-CH-N N, with HÁ Á ÁO aa = 2.51 Å and NÁ Á ÁO aa = 2.8141 (14) Å .

tert-Butyl N-[1-diazoacetyl-3-(methylsulfanyl)propyl]carbamate
T. Mehmood, J. H. Zaidi and P. G. Jones Comment α-Diazocarbonyl compounds find widespread applications in organic and, especially, natural product synthesis (Padwa & Weingarten, 1996). The ready availability, relative stability and facile decomposition of these compounds under various conditions (e.g. thermal, photochemical; acid-, base-and transition-metal-catalysis) make them useful intermediates (Doyle et al. 1998). Furthermore, α-diazoketones undergo a variety of transformations such as cyclopropanation, aziridine formation, ylide formation, C-H or C-X insertion reactions and cyclization reactions (Ye & McKervey, 1994). These reactions are chemoselective, and promote the formation of new carbon-carbon and carbon-heteroatom bonds under mild conditions. Asymmetric versions of diazocarbonyl reactions have been reported to produce enantiomerically pure compounds (Doyle & McKervey, 1997). One such method is the Arndt-Eistert synthesis, which consists of conversion of activated carboxylic acids to diazoketones by the action of diazomethane, followed by Wolf rearrangement. The method has become widely used in recent years for the synthesis of β-peptides and β-amino acid derivatives from appropriately protected α-amino acids (Müller et al. 1998). Here we present the structure of an α-diazocarbonyl compound based on methionine.
The main feature of the molecular packing is the classical H bond N1-H1···O3, which links the molecules via the 3 1 screw operator to form chains parallel to the z axis (Fig. 2). Within the chains, an unusual three-centre interaction is also observed, whereby the carbonyl oxygen O2 is involved in short contacts to H8 and N2 of the diazocarbonyl group of a neighbouring molecule. The former is far from linear (angle 110°) but this is not unusual for three-centre interactions. The latter may be interpreted as a dipole-dipole interaction [dimensions: N2···O2 2.8141 (14) Å, C8-N2···O2 73.5 (1)°]. The remaining "weak" C-H···O interactions (Table 1)  Experimental 10 mmol of BOC-protected methionine was dissolved in 50 ml of dry distilled THF under inert conditions. To maintain basic conditions 12 mmol (1.66 ml) of triethylamine was added. Then 10 mmol (0.95 ml) of ethyl chloroformate was added, and the mixture stirred for 15 min at 248 K. 13 mmol of diazomethane were then added at 268 K and the mixture was further stirred for 30 min. After this temperature was allowed to rise to room temperature over 3 h. The reaction was then quenched with 3-4 drops of glacial acetic acid. The solvent was evaporated under vacuum. The residue was dissolved in ethyl acetate, extracted with aq. solutions of NaHCO 3 and NH 4 Cl and dried over anhydrous MgSO 4 . The crude product was purified by column chromatography (yield 85%; m.p.326-328 K).

Refinement
The NH hydrogen was refined freely. Methyl H atoms were identified in difference syntheses, idealized and refined as rigid groups with C-H 0.98 Å and H-C-H angles 109.5°, allowed to rotate but not tip. Other H atoms were placed in calculated positions and refined using a riding model with C-H 0.98 Å (methylene) or 0.99 Å (methine); hydrogen U values were fixed at 1.5 × U(eq) of the parent atom for methyl H and 1.2 × U(eq) of the parent atom for other C-H. Data are 100% complete to 2θ 145°. The absolute configuration S at C6 (and thus the space group P3 1 rather than its enantiomer P3 2 ) was determined by the Flack (1983) parameter, which refined to 0.023 (10). Fig. 1. The molecule of the title compound in the crystal. Ellipsoids correspond to 50% probability levels.   (14) N2 -O2_$1; 73.5 (1) C8 -N2 -O2_$1; Operator $1 -x + y+1, -x + 1, z -1/3

tert-Butyl N-[1-diazoacetyl-3-(methylsulfanyl)propyl]carbamate
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq