Direct Synthesis of G-2N

Tuning Photoenolization-Driven Cycloadditions Using Theory and Spectroscopy

The first broad spectrum investigation into the photoenolization/Diels-Alder (PEDA) sequence was carried out using M06-2X/6-31+G(d,p) in conjunction with SMD solvation and supported by experimental UV-vis spectroscopy. A test set of 20 prodienes was chosen to examine the role of the H atom acceptor group (substituted and unsubstituted carbonyl, thiocarbonyl, and imine), the H atom donor group, and bystander ring substituents. As reaction partners for the photogenerated dienes, a diverse test set of 20 dienophiles was examined, comprising electron rich, electron poor, neutral, strain activated, hydrocarbon, and heteroatom-containing molecules including CO₂ and CO. A key finding of this work is the demonstration that the PEDA sequence of carbonyl based prodienes is tolerant of most substitution patterns. Another is that thiocarbonyl derivatives should behave analogously to the carbonyls but are likely to do so much more slowly, due to an inefficient intersystem crossing, an endothermic 1,5-hydrogen atom transfer (HAT) step, and a [1,5] sigmatropic H shift to regenerate the starting material that outcompetes the [4 + 2]cycloaddition. In contrast, the T₁ state of the ortho-alkyl imines displays the incorrect orbital symmetry for 1,5-HAT and is correspondingly accompanied by higher barriers, even in the excited state. However, provided these barriers can be overcome, the remaining steps in the PEDA sequence are predicted to be facile. The Diels-Alder reaction is predicted to be of much broader scope than reported synthetic literature: while electron poor dienophiles are expected to be the most reactive partners, ethylene and electron rich alkenes should react at a synthetically useful rate. CO is predicted to undergo a facile (4 + 1)cheletropic addition instead of the normal [4 + 2]cycloaddition pathway. This unique photoenolization/cheletropic addition (PECA) sequence could provide metal-free access to benzannelated cyclopentanones.

Synthesis of Naphthocyclobutenes from α-Naphthyl Acrylates by Visible-Light Energy-Transfer Catalysis

Methyl (α-naphthyl) acrylates bearing an ortho-substituent with a hydrogen atom produce naphthocyclobutenes upon Ir(Fppy)₃-mediated photosensitization. This reaction can be described as a carbon analogue of the Norrish-Yang reaction: upon triplet excitation of the acrylate a 1,5-HAT results in a 1,4-diradical which forms the cyclobutene. Diastereoselectivities up to >20:1 were observed for the ring-closure reaction.

Photochemistry of 2-Alkoxymethyl-5-methylphenacyl Chloride and Benzoate

Irradiation of 2-(alkoxymethyl)-5-methyl-alpha-chloroacetophenones (1a-c) and 2-(methoxymethyl)-5-methylphenacyl benzoate (1d) in dry, nonnucleophilic solvents afforded 3-alkoxy-6-methylindan-1-ones (3a-c) in very high chemical yields. 3-Methylisobenzofuran-1(3H)-one (2) was, however, isolated as a major photoproduct in the presence of trace amounts of water. Quenching experiments and laser flash spectroscopy revealed that the indanone derivatives 3 are formed by 1,5-hydrogen migration from the lowest triplet excited state of the acetophenones 1 and cyclization of the resulting photoenols. In contrast, production of the lactone 2 in wet solvents was found to result from two consecutive photochemical transformations. The photoenols produced by photolysis of 1a-c add water as a nucleophile to form 2-acetyl-4-methylbenzaldehyde (4), which is further converted to 2 via a second, singlet state photoenolization process. Exhaustive photolysis of 1a in methanol produced the acetal 2-(dimethoxymethyl)-5-methylacetophenone (7a) as the exclusive product. The remarkable selectivity of these photoreactions may well be useful in synthetic organic chemistry.

Total Synthesis of Hamigerans and Analogues Thereof. Photochemical Generation and Diels−Alder Trapping of Hydroxy-o-quinodimethanes

A number of naturally occurring substances, including hamigerans, contain ring systems which are fused to an aromatic nucleus. A general and streamlined method for the construction of such benzannulated bi- and polycyclic carbon frameworks has been developed, and its scope and limitations were explored. On the basis of the photoenolization of substituted benzaldehydes and subsequent Diels-Alder (PEDA) trapping of the generated hydroxy-o-quinodimethanes, this method was optimized to set the stage for the total synthesis of several naturally occurring members of the hamigeran class. Specifically, the developed synthetic technology served as the centerpiece process for the successful asymmetric synthesis of hamigerans A (2), B (3), and E (7). In addition to the PEDA reactions, several other novel reaction processes are described, including a high-yielding decarbonylative ring contraction and an oxidative decarboxylation of a hydroxyl beta-keto ester to afford an alpha-diketone. A number of analogues of these biologically active natural products were also prepared by application of the developed technology.

Intramolecular Cyclizations of o-Acylbenzyllithiums. Formation of Benzocyclobuten-1-ol Derivatives and Their Thermal Isomerization

The formation of benzocyclobutenol derivatives by intramolecular cyclizations of o-acylbenzyllithiums is described. Treatment of o-(trialkylsilylmethyl)phenyl ketones with lithium diisopropylamide (LDA) followed by quenching of the resulting benzylic carbanions with chlorotrialkylsilane resulted in stereoselective formation of the corresponding 1-trialkylsiloxy-2-(trialkylsilyl)benzocyclobutenes in good yields. Subsequently, o-acyl-m-methoxybenzyllithiums were found to work well in cyclization to benzocyclobuten-1-ol derivatives. The reaction of 2-benzoyl-3,4,5-trimethoxybenzyllithium, generated in situ by deprotonation of 6-methyl-2,3,4-trimethoxybenzophenone with LDA, with chlorotrimethylsilane afforded the corresponding 1-(trimethylsiloxy)benzocyclobutene. Cyclization of 2-pivaloyl-3-methoxybenzyllithiums, generated in situ from tert-butyl 2-methyl-6-methoxyphenyl ketones upon deprotonation with LDA, proceeded spontaneously even at -78 degrees C to give the corresponding benzocyclobuten-1-ols. We also describe the results of thermal isomerization of these 1-trimethylsiloxy-2-(trialkylsilyl)benzocyclobutenes.