Condensed quinazoline derivatives exhibit a wide range of biological activities. One of the most promising methods for synthesizing condensed quinazoline derivatives is electrophilic intramolecular cyclization, which is widely employed in the preparation of azines and their fused analogues. A special place among quinazolines is occupied by their alkenyl and alkynyl derivatives, which represent bicentered nucleophilic systems, making them convenient model substrates for studying electrophilic intramolecular heterocyclization reactions. In the present study, a series of condensed chalcogen-functionalized thiazoloquinazolines was synthesized, and their growth-regulating activity toward soybean seeds Glycine max (L.) Merrill at the early stages of organogenesis was investigated. Analysis of the experimental data revealed that treatment of soybean seeds with 0.001–0.1% solutions of the chalcogen-containing thiazolo[3,2-a]quinazolines resulted in higher values of growth parameters—such as seedling mass and length—compared to the control (distilled water). Specifically, seedling mass increased by 14–18%, while seedling length increased by 3–19%. An increase in the concentration of the quinazolines to 0.1% led to a pronounced inhibition of growth and organism development. Further investigation of thiazoloquinazolines and their structurally related analogues as growth-regulating agents may contribute to improving the yield and quality of economically significant crops, as well as provide insights into the biochemical processes occurring in plants upon treatment with these compounds.