Purpose: The authors present the first author's recent experience as a design case, capturing approach and principles to transitioning a course from in-person to remote modality while maintaining its collaborative, synchronous and dialogic nature. Design/methodology/approach: A mathematical content course for preservice teachers at a mid-Western university was used as a site for this case study. Data include video recordings of synchronous online sessions, students' written work and their reflections. Using a design case approach, the authors identified new social norms, which were interactively established from the mutual constitution of expectations and commitments between the instructor and students and which gradually became important aspects of the culture of online, whole-class discussions. Findings: Dialogic discussions revealed a variety of student-generated methods to solve problems that enabled student-led interactions to become the center of each session. Opportunities for students to reflect on their own ideas and reasoning about mathematics often occurred in the synchronous settings in which they were allowed to express provisional ideas and clarify their thinking to others. Making connections between relevant educational theories and pragmatic instructional decisions of course instructors and designers in the crisis, this study aims to identify principles of instructional design and implementation that became salient in this case, backing them up with evidence from student voices. Originality/value: The authors discuss how these principles of instructional design and implementation could make synchronous online instruction manageable for more instructors in future where entire semesters may occur through a virtual medium.