Synthesis of Functionalized Bicyclo[3.2.0]heptanes - a Study of the [2+2] Photocycloaddition Reactions of 4-Hydroxycyclopent-2-enone Derivatives

Humilisin E: Strategy for the Synthesis and Access to the Functionalized Bicyclic Core

Humilisin E is a diterpenoid possessing a rare epoxidized cyclononene trans-fused with a bicyclo[3.2.0]heptane core. We have identified the P atropisomer of the corresponding cyclononadiene as a potential biosynthetic/synthetic precursor to humilisin E and reported two different strategies for the stereocontrolled synthesis of the appropriately functionalized bicyclic cores of humilisin E. The first route involves a Stork epoxynitrile cyclization via a Mg alkoxide, and the second, more stereoselective approach utilizes the Wolff rearrangement as the key step.

Intramolecular Alkyne-de Mayo Reaction Using the Enol Methylene Acetal Linker: Experimental Implementation and Computational Simulation

The enol-linked intramolecular alkyne-de Mayo reaction is a photochemically triggered cascade reaction suitable for the synthesis of substituted dihydrotropones by two-carbon ring expansion of enol ethers of cyclopentane-1,3-dion. We report on the implementation of the methylene acetal linker and the isolation of the initial (2 + 2) photocycloadduct in substances. We have investigated in depth the modus operandi of the ring-opening of the π-donor-π-acceptor cyclobutene derivatives by computational chemistry.

An enantiodivergent synthesis of N-Boc-protected (R)- and (S)-4-amino cyclopent-2-en-1-one

Routes are reported for the synthesis of both (1 R)- and (1 S)- tert-butyl-(4-oxocyclopent-2-en-1-yl)carbamate 2. Featuring Mitsunobu reactions with di- tert-butyl iminodicarbonate, both syntheses begin from ( S)-4-[( tert-butyldimethylsilyl)oxy]cyclopent-2-en-1-one (3) and take advantage of the 1,4-cyclopentenyl dioxygenation pattern of this optically active starting material. Thus both (−)- and (+)-2 have been accessed from 3 in an enantiodivergent manner in 11% and 10% overall yield over five and seven reaction steps, respectively.

Acetylene and Ethylene: Universal C2 Molecular Units in Cycloaddition Reactions

Acetylene and ethylene are the smallest molecules that contain an unsaturated carbon–carbon bond and can be efficiently utilized in a large variety of cycloaddition reactions. In this review, we summarize the application of these C2 molecular units in cycloaddition chemistry and highlight their amazing synthetic opportunities.

1 Introduction

2 Fundamental Features and Differences of Cycloaddition Reactions Involving Acetylene and Ethylene

3 (2+1) Cycloaddition

4 [2+2] Cycloaddition

5 (3+2) Cycloaddition

6 [4+2] Cycloaddition

7 (2+2+1) Cycloaddition

8 [2+2+2] Cycloaddition

9 The Use of Acetylene and Ethylene Cycloaddition for Deuterium and 13C Labeling

10 Conclusions

Synthesis of 2-chromanone-fused [3.2.0] bicycles through a phosphine-mediated tandem [3 + 2] cyclization/intramolecular Wittig reaction

A phosphine-mediated tandem [3 + 2] cyclization/intramolecular Wittig reaction of alkynone is described. 2-Chromanone-fused bicyclo[3.2.0]heptenones were synthesized in moderate to high yields with remarkably high regio- and diastereoselectivities.

UV-Curable Biobased Polyacrylates Based on a Multifunctional Monomer Derived from Furfural

The controlled polymerization of a new biobased monomer, 4-oxocyclopent-2-en-1-yl acrylate (4CPA), was established via reversible addition-fragmentation chain transfer (RAFT) (co)polymerization to yield polymers bearing pendent cyclopentenone units. 4CPA contains two reactive functionalities, namely, a vinyl group and an internal double bond, and is an unsymmetrical monomer. Therefore, competition between the internal double bond and the vinyl group eventually leads to gel formation. With RAFT polymerization, when aiming for a degree of polymerization (DP) of 100, maximum 4CPA conversions of the vinyl group between 19.0 and 45.2% were obtained without gel formation or extensive broadening of the dispersity. When the same conditions were applied in the copolymerization of 4CPA with lauryl acrylate (LA), methyl acrylate (MA), and isobornyl acrylate, 4CPA conversions of the vinyl group between 63 and 95% were reached. The additional functionality of 4CPA in copolymers was demonstrated by model studies with 4-oxocyclopent-2-en-1-yl acetate (1), which readily dimerized under UV light via [2 + 2] photocyclodimerization. First-principles quantum mechanical simulations supported the experimental observations made in NMR. Based on the calculated energetics and chemical shifts, a mixture of head-to-head and head-to-tail dimers of (1) were identified. Using the dimerization mechanism, solvent-cast LA and MA copolymers containing 30 mol % 4CPA were cross-linked under UV light to obtain thin films. The cross-linked films were characterized by dynamic scanning calorimetry, dynamic mechanical analysis, IR, and swelling experiments. This is the first case where 4CPA is described as a monomer for functional biobased polymers that can undergo additional UV curing via photodimerization.

Donor–acceptor fluorophores as efficient energy transfer photocatalysts for [2 + 2] photodimerization

Donor–acceptor fluorophores can act as efficient energy transfer photocatalysts to activate enone substrates, realizing photodimerization and isomerization reaction of enone substrates without precious metal photocatalysts.

Development of an Alkyne Analogue of the de Mayo Reaction: Synthesis of Medium-Sized Carbacycles and Cyclohepta[b]indoles

Embedded medium-sized carbacycles and cyclohepta[b]indoles occur frequently as scaffold elements in natural products and bioactive compounds. Described herein is a conceptionally novel photochemically triggered cascade process to these scaffolds. Key to the cascading ring-expansion process is an unprecedented intramolecular alkyne analogue of the de Mayo reaction.

Intramolecular Photocycloaddition of 2(5 H)-Furanones to Temporarily Tethered Terminal Alkenes as a Stereoselective Source of Enantiomerically Pure Polyfunctionalyzed Cyclobutanes

Allyloxymethyloxymethyl and 4-pentenoyloxymethyl substituents have been used as tethering groups to study the intramolecular [2 + 2] photocycloaddition of chiral 5-substituted 2(5 H)-furanones. The photoreactions proceed in good yield and provide the expected regio- and diastereoselective tricyclic compounds with complementary regioselectivity, which depends on whether the vinyl chain is attached to the furanone by an acetal or an ester linkage. Computational simulations agree with experimental observations and indicate that the origin of the different observed regioselectivity in the intramolecular photochemical reaction of lactones 5 and 6 arises from the relative stability of the initial conformers. The synthetic potential of the enantiomerically pure photoadducts is illustrated by preparing an all- cis 1,2,3-trisubstituted cyclobutane bearing fully orthogonally protected hydroxyl groups.

Recent Advances in the Synthesis of Cyclobutanes by Olefin [2 + 2] Photocycloaddition Reactions

The [2 + 2] photocycloaddition is undisputedly the most important and most frequently used photochemical reaction. In this review, it is attempted to cover all recent aspects of [2 + 2] photocycloaddition chemistry with an emphasis on synthetically relevant, regio-, and stereoselective reactions. The review aims to comprehensively discuss relevant work, which was done in the field in the last 20 years (i.e., from 1995 to 2015). Organization of the data follows a subdivision according to mechanism and substrate classes. Cu(I) and PET (photoinduced electron transfer) catalysis are treated separately in sections 2 and 4 , whereas the vast majority of photocycloaddition reactions which occur by direct excitation or sensitization are divided within section 3 into individual subsections according to the photochemically excited olefin.

First total synthesis of the marine natural products clavulolactones II and III

The first total synthesis of the marine prostanoids clavulolactones II and III is presented from an easily accessible chiral, non-racemic cyclopentenone intermediate. Key steps involve selective TBDMS deprotection, selective reduction of the β-side chain and aldol condensation. Clavulolactones II and III were successfully prepared from (S)-4-((tert-butyldimethylsilyl)oxy) cyclopent-2-en-1-one over nine steps, in overall yields of 21 and 7% respectively.

Total synthesis of (±)-hippolachnin A

The first total synthesis of the marine polyketide (±)-hippolachnin A has been achieved in nine linear steps and an overall yield of 9%. Rapid access to the oxacyclobutapentalene core structure was secured by strategic application of an ene cyclization.

Stereoselective cross aldol condensation of bicyclo[3.2.0]alkanones

A cross aldol reaction between [(S)-(-)] or [(R)-(+)]-benzyloxypropanal and silyl enol ethers derived from bicyclo[3.2.0]alkanones was carried out in the presence of TiCl4, leading with total stereoselectivity to a 1 : 1 mixture of enantiomerically pure diastereomers isolated in 81% overall yield. Thus, 5 stereogenic centers could be created starting from one. Furthermore, an efficient access to an enantiomerically pure tricyclo[5.3.0.0(2,6)]decane scaffold was possible via a 4 step reaction sequence.