Size-Induced Inversion of Selectivity in the Acylation of 1,2-Diols.

Relative rates for the Lewis base-catalyzed acylation of aryl-substituted 1,2-diols with anhydrides differing in size have been determined by turnover-limited competition experiments and absolute kinetics measurements. Depending on the structure of the anhydride reagent, the secondary hydroxyl group of the 1,2-diol reacts faster than the primary one. This preference towards the secondary hydroxyl group is boosted in the second acylation step from the monoesters to the diester through size and additional steric effects. In absolute terms the first acylation step is found to be up to 35 times faster than the second one for the primary alcohols due to neighboring group effects.

Size-Driven Inversion of Selectivity in Esterification Reactions: Secondary Beat Primary Alcohols.

Relative rates for the Lewis base-mediated acylation of secondary and primary alcohols carrying large aromatic side chains with anhydrides differing in size and electronic structure have been measured. While primary alcohols react faster than secondary ones in transformations with monosubstituted benzoic anhydride derivatives, relative reactivities are inverted in reactions with sterically biased 1-naphthyl anhydrides. Further analysis of reaction rates shows that increasing substrate size leads to an actual acceleration of the acylation process, the effect being larger for secondary as compared to primary alcohols. Computational results indicate that acylation rates are guided by noncovalent interactions (NCIs) between the catalyst ring system and the DED substituents in the alcohol and anhydride reactants. Thereby stronger NCIs are formed for secondary alcohols than for primary alcohols.

Outcome of dichorionic diamniotic twin pregnancies with spontaneous PPROM before 24 weeks' gestation.

OBJECTIVE: The objective of this study is to examine the outcome in dichorionic diamniotic twin pregnancies with rupture of membranes (PPROM) before 24 weeks' gestation. MATERIAL AND METHODS: Retrospective analysis of fetal and neonatal outcomes in women with spontaneous PPROM before 24 weeks' gestation that were treated at a single tertiary center. RESULTS: Twenty-nine pregnancies fulfilled the inclusion criteria. Mean gestational age at the time of PPROM was 20.4 weeks. Two women opted for termination. The remaining 27 (93.1%) women elected for expectant management. Ten (37.0%) of these delivered before 24 + 0 weeks' gestation. The median gestational age at the time of delivery in the remaining 17 (63%) cases was 26.4 weeks. In those patients that did not deliver within the first 5 days of PPROM, the chance of reaching 24 + 0 weeks was 85%. Co-twins that had PPROM compared with those without PPROM suffered more often from several neonatal complications. Overall, about 40% of the fetuses in the PPROM group and 70% of the non-PPROM group survived without major complications. CONCLUSION: Dichorionic diamniotic twin pregnancies with PPROM prior to 24 + 0 weeks' gestation have a 60% chance of delivering at >24 weeks. Survival rates without major complications in twins with and without PPROM are 40% and 70%, respectively.

Outcome of pregnancies with spontaneous PPROM before 24+0 weeks' gestation.

OBJECTIVE: To examine the contemporary outcome in women with rupture of membranes (PPROM) before 24+0 weeks' gestation. STUDY DESIGN: Retrospective analysis of women with spontaneous PPROM before 24+0 weeks that were treated at the University of Tuebingen/Germany. The search of the database included common maternal and pregnancy characteristics as well as the neonatal outcomes. RESULTS: One hundred and one pregnancies fulfilled the inclusion criteria. 32 (31.7%) women opted for termination of pregnancy, which were excluded from further analysis. The gestational age at PPROM in the 69 women with an expectant management was 21.3 (IQR 19.1-22.6) weeks. 40 (58.0%) pregnancies carried on beyond 24+0 weeks. Multiple regression analysis indicated that the time of PPROM and the absence of oligo-/anhydramnios were associated with a prolongation beyond 24+0 weeks. In the 40 pregnancies that remained intact beyond 24+0 weeks' gestation, the fetuses were born at 27.7 (IQR 25.3-30.9) weeks. Survival without major complications was observed in 22 (55.0%) fetuses. Multiple regression analysis indicated that only the gestational age at the time of delivery was significantly associated with such an intact survival. CONCLUSION: In cases with PPROM there is a 60% chance of a prolongation beyond 24+0 weeks. About half of these fetuses will be discharged alive without major complications.