The crystal structure of the title compound has been determined from X-ray data measured on a four-circle diffractometer. The crystals are monoclinic, space group P21/c, with a= 13·938, b= 12·882, c= 20·220 Å, β= 112·87°, Z= 4. 3459 Reflections were measured of which 2492 exceeded 2·5σ, and were used in the analysis. The structure was solved by heavy-atom methods and refined by least-squares procedures using large blockdiagonal approximations to R 0·063. The macro-ring is not strictly planar; ring A in particular bends out of the mean plane of the ring system; the β-atoms deviate by 0·67–0·69 Å from the plane defined by the four nitrogen atoms. The bromine ion forms three hydrogen bonds with pyrrole nitrogen atoms, N(21), N(22), and N(23), as well as a fourth one with C(39) of a chloroform molecule. A second molecule of chloroform is disordered.

An intermediate obtained by Lowe and Ramsay in the synthesis of a homopenicillin has been shown by X-ray analysis to have a structure including a β-lactam ring fused to a thiomorpholine ring carrying 2,2-dimethyl and 3-benzyloxycarbonyl substituents. The compound crystallises in the space group P21/c, with Z= 4 in the unit cell of dimensions a= 19·160, b= 6·369, c= 19·052 Å, β= 111·59°. The crystal structure was solved from diffractometer data by the symbolic addition method and refined to an R value of 5·16% for 2697 independent reflections by iterative block-diagonal least-squares procedures. The central six-membered ring exists in a distorted chair form and is oriented at 140·8° to the fused β-lactam ring.