The C 5 -hydroperoxyenals (C 5 -HPALDs) are a newly-recognized class of multi-functional hydrocarbons produced during the hydroxyl radical (OH)-initiated oxidation of isoprene. Recent theoretical calculations suggest that fast photolysis of these compounds may be an important OH source in high-isoprene, low-NO regions. We report experimental constraints for key parameters of photolysis, OH reaction and ozone reaction of these compounds as derived from a closelyrelated, custom-synthesized C 6 -HPALD. The photolysis quantum yield is 1.0 AE 0.4 over the range 300-400 nm, assuming an absorption cross section equal to the average of those measured for several analogous enals. The yield of OH from photolysis was determined as 1.0 AE 0.8. The OH reaction rate constant is (5.1 AE 1.8) Â 10 À11 cm 3 molecule À1 s À1 at 296 K. The ozone reaction rate constant is (1.2 AE 0.2) Â 10 À18 cm 3 molecule À1 s À1 at 296 K. These results are consistent with previous first-principles estimates, though the nature and fate of secondary oxidation products remains uncertain. Incorporation of C 5 -HPALD chemistry with the above parameters in a 0-D box model, along with experimentally-constrained rates for C 5 -HPALD production from isomerization of first-generation isoprene hydroxyperoxy radicals, is found to enhance modeled OH concentrations by 5-16% relative to the traditional isoprene oxidation mechanism for the chemical regimes of recent observational studies in rural and remote regions. This enhancement in OH will increase if C 5 -HPALD photo-oxidation products also photolyze to yield additional OH or if the C 5 -HPALD production rate is faster than has been observed.

Structurally novel endoperoxides can be sythesized by the photocatalytic cyclotrimerization of bis(styrene) substrates with molecular oxygen. The optimal catalyst for this process is Ru(bpz)32+, which is a markedly more efficient catalyst for these photooxygention reactions than conventional organic photosensitizers. The 1,2-dioxolane products are amenable to synthetic manipulation and can be easily processed to 1,4-diols and γ-hydroxyketones. An initial screen of the biological activity of these compounds reveals promising inhibition of cancer cell growth.

A total synthesis of brevetoxin A is reported. Two tetracyclic coupling partners, prepared from previously reported advanced fragments, were effectively united via a Horner-Wittig olefination. The resulting octacycle was progressed to substrates that were explored for reductive etherification, the success of which led to a penultimate tetraol intermediate. The tetraol was converted to the natural product through an expeditious selective oxidative process, followed by methylenation.The exquisite structures of marine polycyclic ether natural products have captured the imagination of synthetic chemists for over two decades. The structures of the polyether ladder toxins characteristically contain a linear series of trans fused ether rings of varying sizes from five to nine members with assorted methyl and hydroxyl substituents appended. As novel technologies for the convergent preparation of these targets have emerged, a number of total syntheses of the ladder toxins have been completed. 1 The structure of brevetoxin A (1), a representative member of this class, was first elucidated in 1986 by Shimizu 2a,b , and coworkers by X-ray analysis and independently determined by Nakanishi through spectroscopic studies. 2c Brevetoxin A (1) contains ten rings (including five-, six-, seven-, eight, and ninemembered oxacycles) fused in a linear array adorned by 22 tetrahedral stereocenters. A metabolite of Karenia brevis, brevetoxin A is a toxic component of the infamous red tide phenomenon, which has been responsible for massive fish kills as well as neurotoxic shellfish poisoning and bronchial irritation in humans. 3 The potent activity of brevetoxin A is attributed to strong binding to the α subunit of the voltage-sensitive sodium ion channels effecting an increase in the mean channel open time and inhibiting channel inactivation. The planned approach for the total synthesis of brevetoxin A 5 focused on a versatile endgame that would exploit the selective manipulation of tetraol 2 (Scheme 1), which would derive from mixed methyl ketal 3 via stereoselective reductive etherification. Ketal 3 would be obtained through the stereoselective Horner-Wittig coupling 6 of phosphine oxide 5 and aldehyde 6. This route was attractive not only because it allowed for optimal convergence by simplifying the natural product into two halves of similar complexity, but also because it found precedent in the strategy previously reported by Nicolaou. 4 Further, it was reasoned that the dithioketal moiety of aldehyde 6 could serve as a stabilized precursor to mixed ketal 3, or lead to sulfone 4 in the event that formation or reductive etherification of mixed ketal 3 proved problematic. The Horner-Wittig coupling partners 5 and 6 would be obtained from advanced fragments 7 and 8, respectively. The BCDE fragment 7 and GHIJ subunit 8 had been previously prepared in significant quantities through similar highly convergent [X+2+X] strategies based on a Horner-Wadsworth-Emmons coupling of the B and E ring units (and the G and J subunits) and subs...

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