Photochemical Deracemization of Allenes and Subsequent Chirality Transfer

Trisubstituted allenes with a 3‐(1′‐alkenylidene)‐pyrrolidin‐2‐one motif were successfully deracemized (13 examples, 86–98 % ee) employing visible light (λ=420 nm) and a chiral triplet sensitizer as the catalyst (2.5 mol %). The photocatalyst likely operates by selective recognition of one allene enantiomer via hydrogen bonds and by a triplet‐sensitized racemization process. Even a tetrasubstituted allene (45 % ee) and a seven‐membered 3‐(1′‐alkenylidene)‐azepan‐2‐one (62 % ee) could be enantiomerically enriched under the chosen conditions. It was shown that the axial chirality of the allenes can be converted into point chirality by a Diels–Alder (94–97 % ee) or a bromination reaction (91 % ee). Ring opening of the five‐membered pyrrolidin‐2‐one was achieved without significantly compromising the integrity of the chirality axis (92 % ee).     

Enantioselective crossed intramolecular [2+2] photocycloaddition reactions mediated by a chiral chelating Lewis acid

In intramolecular [2+2] photocycloaddition reactions, the two tethered olefins can approach each other in a straight or in a crossed fashion. Despite the fact that the latter reaction mode leads to intriguing, otherwise inaccessible bridged skeletons, there has so far not been any enantioselective variants thereof. This study concerned the crossed [2+2]-photocycloaddition of 2-(alkenyloxy)cyclohex-2-enones to bridged cyclobutanes. It was found that the reaction could be performed with high enantioselectivity (80–94% ee) under visible light conditions when employing a chiral rhodium Lewis acid as a catalyst (2 mol%).

An enantioselective crossed [2+2] photocycloaddition is presented which proceeds under visible light irradiation in the presence of a chiral Lewis acidic metal complex. Chelation of two oxygen atoms to the metal centre accounts for the observed enantioface differentiation.     

Total Synthesis of Pulvomycin D

A synthetic route to the pulvomycin class of natural products is presented, which culminated in the first synthesis of a pulvomycin, pulvomycin D. Key elements of the strategy include a pivotal aldol reaction which led to bond formation between the C24-C40 and the C8-C23 fragment. The remaining C1-C7 fragment was attached by a Yamaguchi esterification completing the assembly of the 40 carbon atoms within the main skeleton. Ring closure to the 22-membered lactone ring was achieved in the final stages of the synthesis by a Heck reaction. The completion of the synthesis required the removal of six silyl protecting groups in combination with olefin formation at C26-C27 by a Peterson elimination.     

Determination of Trace Alkali and Alkaline Earth Metals in Orange Juice Using the DC Plasma and the Inductively Coupled Plasma Atomic Emission Techniques

Matrix effects in plasma spectroscopy have been measured in a simulated biological substrate. The authors show comparisons of such effects using both inductively coupled and direct current plasmas. The matrix influences are much more pronounced in direct current plasmas, but are also present when dealing with alkali metals in inductively coupled plasmas as well. The latter finding represents a particularly significant finding because it points the way to a method of determining alkali metals at levels of parts per billion.     

The synthesis of (+)-preussin and related pyrrolidinols by diastereoselective Paternò-Büchi reactions of chiral 2-substituted 2,3-dihydropyrroles

The N-alkoxycarbonyl substituted 2,3-dihydropyrroles 3 and 8 are converted to 2-benzyl-3-pyrrolidinols by the Paternò - Büchi reaction followed by hydrogenolysis. Since the addition of the photoexcited benzaldehyde at the unsaturated heterocycle proceeds in a syn fashion, the benzyl group at C-2 and the hydroxy group at C-3 of the product are cis oriented. The simple and facial diastereoselectivities of the Paternò-Büchi reaction were studied more closely and the relative configuration of the products was elucidated. The thermodynamically less stable endo product is formed as a result of simple diastereoselection. The face differentiation in 2-substituted 2,3-dihydropyrroles is presumably due to the nonplanarity of these heterocycles, which forces attack of the carbonyl group on the face with the existing substituent. All-cis-pyrrolidinols are consequently formed after hydrogenolysis. Following this route, a total synthesis of the pyrrolidinol alkaloid (+)-preussin (1) was conducted, which yielded the target compound in a total yield of 11% over nine steps starting from L-pyroglutaminol (11).     

A reactivity/affinity switch for parallel kinetic resolution: alpha-amino acid quasienantiomers and the resolution of cyclopropene carboxylic acids

A new type of parallel kinetic resolution (PKR) is reported in which quasienantiomers with very similar reactivities give products whose chromatographic properties diverge upon the addition of fluoride. This concept of a reactivity/affinity switch is applied to the PKR of cyclopropene carboxylic acids with all-carbon quaternary centers. This is the first application of alpha-amino acid quasienantiomers in PKR, and it is a complementary approach for acyltransfer systems where the asymmetry is induced by the nucleophile rather than the leaving group. Excellent diastereoselectivities (ranging from 90:10 to 99.5:5) and good yields were obtained for both quasienantiomeric products, and the reactions can be run on significant scale because the separation is trivial. High-level DFT calculations (B3LYP functional with the 6-31+G(d,p) basis set) provided transition-state structures with relative energies that are in accord with the experimental observations.     

Enantioselective [6pi]-photocyclization reaction of an acrylanilide mediated by a chiral host. Interplay between enantioselective ring closure and enantioselective protonation

The [6pi]-photocyclization of the anilides 1a and 5 was studied in the absence and in the presence of the enantiomerically pure chiral lactam 4. The relative configuration of the products was unambiguously established by single-crystal X-ray crystallography and by NMR spectroscopy. A significant enantiomeric excess was observed upon reaction of compound 1a to its photocyclization products at -55 degrees C employing lactam 4 as a chiral complexing agent in toluene as the solvent (66% yield). The trans product ent-3a was obtained in 57% ee, and the minor diastereoisomer (trans/cis = 73/27), cis product ent-2a, was obtained in 30% ee. DFT calculations were conducted modeling the complexation of intermediates 8 and ent-8 to host 4. In agreement with steric arguments concerning the conrotatory ring closure of 1a, the formation of ent-8 is favored leading to the more stable complex 4.ent-8 as compared to 4.8. Whereas the enantioselectivity in the photocyclization to trans compound ent-3a increased upon reduction in the reaction temperature, the enantiomeric excess in the formation of cis compound ent-2a went through a maximum at -15 degrees C (45% ee) and decreased at lower temperatures. Deuteration experiments conducted with the pentadeuterated analogue of 1a, d(5)-1a, revealed that the protonation of the intermediates 8 and ent-8 is influenced by chiral amide 4. In the formation of ent-3a/3a, both the enantioselective ring closure and the enantioselective protonation by amide 4 favor the observed (6aS,10aS)-configuration of the major enantiomer ent-3a. In the formation of ent-2a/2a, the enantioselective ring closure (and the subsequent diastereoselective protonation) favors the (6aR,10aS)-configuration that is found in compound 2a. Contrary to that, the enantioselective protonation by amide 4 shows a preference for ent-2a with the (6aS,10aR)-configuration.