Effects of sidechain length and composition on the kinetic conversion and product distribution of vitamin D analogs determined by real-time NMR

Abstract

Novel pregna-5, 7-dienes were synthesized and subjected to UVB irradiation to generate the corresponding pre-D intermediates, tachysterol and lumisterol analogs. The kinetics of the conversion from each of the pre-D intermediates to the corresponding novel D analogs was investigated by using real time 1H NMR measurements inside the NMR magnet. Both the length and composition of the side chains were found to affect the rate of the kinetic conversion from pre-D intermediates to the thermodynamically more stable D analogs. Compound 7cc which has both a long side chain and a tertiary alcohol moiety showed the highest conversion rate, while compound 4a-S which has a very short side chain without the tertiary alcohol had the lowest conversion rate among the 13 tested compounds. We also determined product distributions for these 5,7-dienes upon UVB irradiation followed by thermodynamic equilibration. No clear correlations between product distribution and side chain length or composition were identifiable under the current experimental conditions, suggesting there are other factors affecting the kinetics during the photochemical reactions for these 5,7-dienes. To the best of our knowledge, this is the first time the influences of side chain length and composition on the real time conversion kinetics from pre-D to D are studied. This study could serve as step-stones in future kinetic studies of novel biologically active 5,7-dienes and their corresponding D analogs under more physiologically relevant ex vivo or in vivo conditions, as well as providing important insights into optimizing yields of the desired active products during their organic syntheses.

Keywords: UVB; kinetics; pre-D; real-time NMR; vitamin D.

Figure 1. UVB induced transformation of 5,7-dienes to the D, T, and L-like analogs.

Figure 2. 1.8 mM of each compound solution in methanol-d₄ was irradiated for 5 min by UVB (280–315 nm). Each irradiated sample was monitored by proton NMR at 37°C immediately after the irradiation in a time course manner. Data was recorded for each time point of 0 min, 10 min, and every 30 min thereafter.

Figure 3. Comparison of conversion from pre-D analogs to D-like analogs over time for 4a-R, 8a, and 7cc.

Figure 4. Synthesis of 5,7-dienes. Step a is for 1a, 1d only; step d is for 3(a–c) only; step 3 is for 3c only; step f is for 3d only; step g is for 3c only. Reagents and conditions: (a) Ac₂O, microwave, p-toluenesulfonic acid monohydrate; (b) dibromantin, 2,2'-azobisisobutyronitrile, benzene/hexane (1:1), 100°C, reflux; (c) Bu₄NBr, Bu₄NF, THF, rt; (d) LiAlH₄, THF, 0°C; (e) K₂CO₃, MeOH-THF, Argon bubbling, rt; (f) K₂CO₃, MeOH-THF, air, rt; (g) R-Li (7ca,7cd) or R-MgBr (7cb-mL), THF; (h) DBU, THF, MeI.