Achiral space group

Hasegawa et al. reported another example of a [2 + 2] asymmetric transformation in a chiral crystal (Scheme 5) [13]. Ethyl 4-[2-(pyridyl)ethenyl] cinnamate 8b crystallizes in a chiral space group P2 2 2 and upon irradiation yields a chiral dimer with 92-95% ee however, the methyl and n-propyl esters (8a and 8c) crystallized in achiral space groups. 

The compounds in this report usually contain a chirotopic stereogenic carbon ring atom, and were prepared as racemic mixtures. Hypothetically, if the BC conformation prevails, then one can imagine two enantiomers in solution (reference, 5)-BC 9 and (retro-inverso,R)-BC 9-bar. Since this stereochemistry is complicated, it will be helpful if we refer to the descriptor for only one enantiomer. Therefore, in an arbitrary but consistent manner in this report, we will always define the reference ring chirality and label tropicity to be that of the (S)-enantiomer. For example, suppose a racemic mixture of (reference,S)-BC 9 and (retro-inverso,R)-BC 9-bar affords crystals belonging to an achiral space group so that both enantiomers in the racemic compound are present in the crystal lattice. Let us further suppose that dissolution of these crystals will give the same solution-state conformation. We will write that the solid-state (reference,S)-BC 9... 

Racemic (1 / A,3RiS )-3-methyl-1-phenyl-3,4,5,6-tetrahydro-l//-2,5-benzoxazocine-5-carbonitrile (35) crystallized as a racemic compound in an achiral space group.1 X-ray structure analysis showed that one of the enantiomers in the crystal was bent into the (retro,S)-TBC conformation (for torsion angles of the X-ray structure, see Fig. 18).1 TBC is a relatively rare d.v-cyclooctene conformation, and is the fourth and highest energy conformation for d.v-cyclooctene within an energy window of 33.4 kJ from the ground state.1 Selected FI and 13C solution-state and 13C cp/mas... 

The racemic mixture of 36 crystallized as a racemic compound in an achiral space group crystal. Analysis of its X-ray crystallographically determined structure shows one of the enantiomers to have a reference,S)-TCBtype33 conformation as depicted in the iconic drawing for 36 (see Fig. 19). Conformational families exist for cis-3 or trans-44 cyclononenes. The various members of a family maintain similar torsion angles for six of the nine bonds compared to those measured for their cyclononane saturated parent.3 Double bonds are formed from either synclinal or antiperiplanar-type bonds in the parent. Conversion of a single bond to a double bond will usually... 

The racemic mixture of 6,6-dimethyl-l-phenyl-l,3,4,5,6,7-hexahydro-2,6-ben-zoxazonium iodide (39) crystallized as an anhydrate racemic compound in a monoclinic achiral space group crystal. The racemic mixture of the corresponding... 

The racemic mixture of (1 / .S 3.S / )-3-methyl-1-phenyl-1,3,4,5,6,7-hcxahydro-2, 6-benzoxazonine-6-carbonitrile (43) crystallized as a racemic compound in an achiral space group crystal.3 CA-to-phenyl substitution of a methyl at C(3) did not change the solid-state (reference,S)-SCBtype2 conformation of the parent nine-membered... 

Honeycomb layer stacking for 2D compounds crystallizing in (a) the R3c achiral space group, and (b) in the P63 chiral space group. Only the first atoms of the alkyl chains around the nitrogen atom of the [N(n-C4H9)4]+ cation are shown. 

Gautham. N. A conformational comparison of crystallo-graphically independent molecules in organic crystals with achiral space groups. Acta Crystallogr., Sect. B 1992. 48,... 

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