The structure of the title compound was determined by the Patterson method and refined to anR of 0·10 for 8065 observed reflections. The crystals are triclinicP1,a=12·408(2),b=13·157(2),c=14·479(8)Å, α=74·706(8), β=88·728(9), γ=88·660(13)° withZ=4. The intensity data were collected on an Enraf-Nonius CAD-4 single crystal X-ray diffractometer. The benzoxazaphosphorine ring adopts a conformation intermediate between a half-chair and a sofa with the phosphoryl oxygen O(2) in an axial and the 2-chlorophenyl andt-butyl groups which aretrans to each other in equatorial positions. The cyclohexyl ring adopts a chair conformation with the nitrogen atomN(5) in an equatorial position. In one conformation of the molecule (I) thet-butyl group atoms and one of the carbon atoms, C(24) in the cyclohexyl ring are disordered. In the other conformation (molecule II) the 2-chlorophenyl ring adopts a conformation midway between a half-chair and a sofa. The molecular packing involves mainly van der Waals contacts.

C₁₀H₁₀O₃, monoclinic,P2₁/c,a=15·071(10),b=5·435(9),c=16·058(10) Å, β=129·57 (10)^o,V=1013·7ų,Z=4,Dm=1·17 gm/cm³,Dc=1·16 gm/cm³, λ (CuK_α)=1·5418Å, μ=6·8 cm⁻¹,Mr=178·2. The structure was refined toR=0·10 for 637 observed reflections. The intensity data were estimated visually by the equi-inclination multiple film Weissenberg technique withb as the rotation axis using Ni-filtered CuK_α radiation. The interplanar angle between the carboxyl group and the benzene ring is 81(1)^o. The molecules are linked in dimers around a centre of symmetry by O−H…O hydrogen bonds (2·76Å).