The first synthesis of highly substituted 3-alkyl-oxetan-2-ylidenes from allenoates was developed by using the bicyclic guanidine 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as an exceptionally active nitrogen Lewis base catalyst.

5, was discovered as an excellent catalyst for the Morita-BaylisHillman reaction for previously hard-to-activate α,γ-dialkyl allenoate substrates. The obtained densely substituted allenic alcohols, which are generally inaccessible with other Lewis base catalysts, could be further converted into 2,5-dihydrofuran and 2H-pyran-2-one heterocyclic structures with challenging substitution patterns.In organic chemistry, 'superbases' are most commonly defined by their Brønsted basic properties. 1 While more thorough definitions do in fact exist, 2 a superbase is often simply regarded as any species of which the corresponding acid can no longer be easily deprotonated by the hydroxide ion OH -. Considering only the organic, that is, nonmetal superbases, guanidines have emerged as especially versatile reagents and organocatalysts for synthetic organic chemistry. 3 While the employment of the Brøn-sted basic properties of guanidines is thus quite firmly established nowadays, applications of their pronounced Lewis basic properties are still in their infancy. 4 Especially bicyclic guanidines, 5 however, have recently been recognized as not only strongly Brønsted basic, but also remarkably Lewis basic and highly nucleophilic reagents. 6 We herein report a further application of these 'super' Lewis basic properties of guanidines for the nucleophilic activation of densely substituted allenoates.Lewis basic allenoate activations and their applications in organic synthesis have undergone a staggering development since their initial discovery in 1995. 7,8 Most commonly, tertiary phosphines are employed as the nucleophilic catalysts due to their high nucleophilicity and Lewis basicity. 9 In general, the initial attack of a nucleophilic catalyst on an allenoate ester A generates zwitterionic dienolate intermediates B/B′, which behave as nucleophiles at the α-and/or γ-positions (Scheme 1). While this dienolate reactivity is readily exploited in the case of buta-2,3-dienoates, that is, allenoates without any further alkyl substituents, the situation becomes much more complex in the case of α-and/or γ-substituted allenoates. γ-Methyl-derived dienolates B can thus undergo an umpolung reaction by a proton shift to form vinyl ylides C with nucleophilic properties at the β-and δ-positions. Scheme 1 Potentially undesirable umpolung reactions after nucleophilic activation of α-or γ-methyl allenoates by phosphine catalysts. Nucleophilic positions are shown in red.Similarly, an α-methyl allenoate derived dienolate B′ could be transformed into a methylene ylide C′, which is nucleophilic at the β-and β′-positions. While both umpolung reactions have been exploited synthetically, 10,11 they also represent a major limitation on the use of dienolate reactivity for substituted allenoate substrates. The use of nitrogen Lewis bases (e.g. DABCO) could theoretically prevent unwanted umpolung reactions and allow for dienolate reactivity even in alkyl-substituted cases. Unfortunately, however, tertiary amines generally proved insufficiently reactive for the...

A detailed account of the synthesis of chiral bicyclic guanidinium salts is presented. This work represents the first systematic investigation of an approach toward the challenging target molecules via a key guanylation step employing di(imidazole-1-yl)methanimine (6) followed by a two-fold cyclization, which resulted in guanidinium salts 8 and 10. Factors governing the regioselectivity of the final cyclization step are discussed based on further data obtained in the course of the attempted syntheses of two additional bicyclic guanidinium salts.

The bicyclic guanidine 1,5,7-triazabicyclo-A C H T U N G T R E N N U N G [4.4.0]dec-5-ene (TBD) was discovered as an efficient catalyst for the reaction of g-substituted allen-A C H T U N G T R E N N U N G oates with aromatic aldehydes. 4H-1,3-Dioxin-6-ylpropanoates with four newly formed bonds and four stereogenic centers were obtained in good yields and excellent diastereoselectivities by two consecutive Morita-Baylis-Hillman reactions, acetalization and intramolecular Michael addition. This four-step reaction cascade not only significantly expands the scope of catalytic allenoate functionalizations but also highlights the potential of TBD to act as a multifunctional Lewis base catalyst.

This feature focuses on a reagent chosen by a postgraduate, highlighting the uses and preparation of the reagent in current research 894 spotlight 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD) as a Lewis Base Compiled by Aleksej Turočkin Aleksej Turočkin was born in 1986 in Nizhny Tagil, Russian Federation. He studied chemistry at Technische Universität München and carried out his master's thesis under the supervision of Professor Dr. Lukas Hintermann in 2011. After a research internship at Priaxon AG, he started his PhD in 2012 in the research group of Dr. Philipp Selig at RWTH Aachen University. His research focuses on the investigation of nucleophilic properties of guanidines and on the syntheses of chiral bicyclic guanidine organocatalysts.