The directors of Startup A have seven key duties.
- Duty to act within powers
- Duty to promote the success of the company
- Duty to exercise independent judgment
- Duty to exercise reasonable care, skill and diligence
- Duty to avoid conflicts of interest
- Duty not to accept benefits from third parties
- Duty to declare interest in proposed transaction or arrangement
If the sale was an attempt to plunder the resources of A in order to enrich themselves as investors in B, the sale would violate most of these duties but most obviously the duty to exercise reasonable care and to avoid conflicts of interest. The minority investors would have an incredibly solid case if they were defrauded by the directors of A in such a blatant way.
On the other hand, if the sale was an arm's length transaction, it is entirely possible that it would be legitimate. It is entirely possible that A would spend $5 million developing a bunch of IP that turns out to only be worth $500,000. It would be entirely reasonable for the directors of A to discover this fact and decide that the most prudent decision was to sell of the IP, pay off the debt, and shut down the company because they didn't believe that the alternative was running up additional debt/ raising additional funds that they wouldn't be able to repay. Investing in early stage companies is risky after all-- sometimes ideas turn out to be not quite as profitable as you had hoped.
Whether it was an arm's length transaction or an attempt to plunder A to enrich the majority shareholders at the expense of the minority would require a judge to look at the actual circumstances. If A and B are largely owned by different sets of people, the directors of A that owned shares in B all declared their interest (and recused themselves from the vote if their stake was large) and there are a ton of emails going back months where the directors are all talking about whether or not there is a path to profitability, that would very much look like a reasonable transaction. If the spouses of the majority shareholders of A own 100% of B, they all hid their interest, and there are a bunch of emails where they talked about how they managed to fleece the 49% shareholders of A, the judge would find the transaction illegitimate.