Preparation of a functionalized tetracyclic intermediate for the synthesis of rhodexin A.

An efficient synthesis of the tetracyclic steroid core, 19, of rhodexin A and sarmentogenin is reported. An initial inverse-electron-demand Diels-Alder reaction of the acyldiene 6 with the silyl enol ether 7a gave the cycloadduct 8 with the required four contiguous stereocenters in a single step. This compound was then transformed into the methylated enedione 13 which afforded after a reductive alkylation and annulation sequence the tetracycle 19.

Synthesis of highly substituted cyclohexenes via mixed Lewis acid-catalyzed Diels-Alder reactions of highly substituted dienes and dienophiles.

[reaction: see text] A high-yielding method is described for the rapid synthesis of very hindered cyclohexenes by the Diels-Alder reaction of hindered silyloxy dienes and dienophiles using the mixed Lewis acid catalyst system (AlBr(3)/AlMe(3)). Thus, reaction of the hindered diene 4 with various substituted enones gave good yields of the expected cycloadducts even though both partners are quite sterically hindered.

Synthesis of the [5-7-6] tricyclic core of guanacastepene A via an intramolecular Mukaiyama aldol reaction.

[reaction: see text] A synthesis of the tricyclic [5-7-6] skeleton of guanacastepene A is described. The six-membered ring of guanacastepene A was constructed by a Diels-Alder reaction. After several functional group transformations, it was coupled to beta-iodocyclopentenone.

Liquid chromatographic determination of fluoroquinolones in egg albumen and egg yolk of laying hens using fluorometric detection.

A liquid chromatographic method was developed for the determination of ciprofloxacin, enrofloxacin, and sarafloxacin at 10-200 ppb in both egg yolk and egg albumen of laying hens. Egg yolk or albumen was acidified with 1 M phosphoric acid followed by deproteination with acetonitrile and centrifugation. The supernate was pipetted out, and the remaining protein pellet was extracted three times with acetonitrile.