The correct title of this article is [4+3] cycloaddition. The substitution or omission of any < > [ ] { } is because of technical restrictions.
A [4+3] Cycloaddition is a cycloaddition between a 4 atom pi-system and a 3 atom pi-system to form a seven-membered ring. Allyl or oxyallyl cations (propenylium-2-olate) are commonly used 3 atom pi-systems, while a diene (such as butadiene) plays the role of the 4 atom pi-system. It represents one of the relatively few synthetic methods available to form seven-membered rings stereoselectively in high yield.[1] The reaction is a topic academic research.